A dropshipping API for a moulding manufacturer: 80% of partner orders with no manual re-entry

A manufacturer of finishing mouldings wanted partner stores to be able to sell its products without holding their own stock, with orders going straight into the shipping system. We built a dropshipping API on WooCommerce that calculates B2B discounts on its own, checks stock and passes orders to Sellasist accurate to the cent. Two months in, around 80% of all the store’s orders come through the API.

The client makes skirting boards, cornices and wall mouldings in polystyrene, MDF (including fire-resistant and moisture-resistant versions), XPS and stainless steel. It sells through its own WooCommerce store and handles orders, shipping and invoices in Sellasist. Companies that sell finishing materials themselves wanted these mouldings in their range, but without keeping stock and without re-entering every order by hand.

Starting point

Dropshipping looks simple: the partner sells, the manufacturer ships. The devil is in the details, and there were plenty of them here:

• B2B prices are individual. Companies with a Business account have separate discounts for individual product categories and groups. A partner cannot type the price into the order, because then the partner controls the discount, not the manufacturer.
• Some products are sold by the metre. In the store, a moulding’s price is per metre, but the partner receives a piece of a specific length. The price per piece has to follow from the product’s length.
• Orders have to land in Sellasist straight away. That is where the shipping label, invoice and payment terms are created. Anything that reaches Sellasist wrong has to be fixed by hand.
• Partners use different systems. One has an online store, another an ERP. Each names shipping methods differently and retries sending an order differently when something goes wrong with the network.

The goal: the partner connects its system once, and from then on orders flow on their own, with the right price, the right shipping and the right payment terms.

The solution: a dropshipping API inside WooCommerce

Instead of a separate platform, we wrote a plugin for the existing store. That way the catalogue, stock and prices have a single source: the same one the store uses. Each partner gets its own API key, which we store in the database only as a hash, so even a database leak doesn’t reveal the keys.

The most important one is POST /orders. Before an order is created, it goes through several gates. See what happens in each scenario:

// the API response will appear here

The price is always calculated by the manufacturer

This was the project’s most important rule. In an order, the partner only sends the SKU and quantity. Even if it sends a price, the system ignores it and calculates its own:

• The partner discount is a single percentage for the whole catalogue, set in the admin panel.
• The Business discount applies when the partner’s email matches a Business account in the store. The plugin links these accounts automatically when the partner is saved. The discount is calculated per product: first by the product group (tag), then by category.
• The winner is the higher of the two discounts. For mouldings sold by the metre, the price per metre is multiplied by the length of the piece, exactly as in the store’s cart.

In practice, partners get their discount from the Business account: 356 of 360 API orders received it automatically, with no manual calculation.

Four things you won’t find in the documentation

The API itself is half the work. The other half began when real orders started flowing into Sellasist. Each of the problems below only showed up in production, and each would have cost the client’s team daily manual fixes.

1. Where the grosz goes missing

Sellasist reads the unit price from the order and requires unit price times quantity to equal the line value exactly. Standard WooCommerce calculates VAT on the net total of the whole line and only rounds at the end. With larger quantities the two methods drift apart by a few grosz, and then Sellasist splits the line into two. With mouldings this is not an exception: 327 of 393 lines in partner orders have a quantity greater than 1.

The fix: we calculate the gross unit price first, multiply it by the quantity and derive VAT as the difference. We set taxes manually on every line and on shipping, so WooCommerce doesn’t recalculate them its own way afterwards.

2. Courier or parcel locker?

At first, all partner orders had the same shipping method in WooCommerce. Sellasist maps shipping by method ID, so it couldn’t tell a courier from a parcel locker, and orders arrived with a message that no shipping method was mapped. It also turned out that free text in the order is not enough: Sellasist builds its mapping list from the methods registered in the store’s shipping zone.

So we added five real shipping methods (courier, parcel locker, personal pickup, free and catch-all), visible in the store’s API but hidden from customers in the cart. The partner can send the shipping type directly or just its name, and the system works out what it means. The shipping cost comes from the same price list that Sellasist uses. After the rules were set up once in its panel, Sellasist also remapped earlier orders.

3. An invoice marked paid before anyone paid

One partner, which places orders straight from its ERP system, reported that its invoices were arriving in Sellasist as paid, even though it has 30 days to pay. The cause was in WooCommerce: setting an order to “processing” automatically sets a paid date. Sellasist saw that date and treated the invoice as paid, skipping the payment terms from the buyer group. The fix is one line that clears the paid date after the status is set. Finding it meant comparing exactly what Sellasist reads from the order.

4. The partner’s order number where people look for it

The warehouse and customer service work in Sellasist, not in WooCommerce. The order number from the partner’s system was saved in the order data, but Sellasist didn’t show it. Now it goes in as the first line of the order note (“Partner no.: …”), which Sellasist displays right away. When a partner calls about a parcel, their number is all it takes.

Documentation for partners and for the team

We prepared two PDF documents for the API. For partners: connection and key, catalogue, placing orders, statuses, error codes, terms of cooperation and a ready-made example in Python. For the client’s team: adding and editing partners, handling orders, discounts, the API log and answers to common questions. The API log records every call, so a “it never arrived on our side” question can be checked in a minute.

Results

The first partner tested the API from July and switched to it fully at the end of August. Since then, partner orders have made up most of the store’s sales.

• 361 orders via the API worth PLN 121,677 gross, 3,682 pieces of goods.
• Around 80% of all store orders in this period came from partners (361 of 453).
• 5,444 API calls in 30 days: 4,033 catalogue requests, 1,176 stock requests and 235 new orders.
• 0 rejected orders and 0 duplicates in the same period. The partner pulls stock levels continuously, so orders are placed for goods that are actually in the warehouse.
• Discounts with no manual calculation: 356 of 360 orders with an automatic Business discount.
• Shipping mapped in Sellasist for all new orders, with no missing-shipping-method message.

Further partners connect on the same terms: they get a key, the documentation and, from day one, the same prices, stock and rules as the first one.

We build similar integrations between online stores and ERP, CRM and shipping systems as part of our ERP/CRM integrationsservice. If you’re wondering what else can be automated in B2B sales, take a look at our articles on automating a B2B sales department i on automation in wholesale.