> ## Documentation Index
> Fetch the complete documentation index at: https://docs.eykdata.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Overview

> Deliver orders, customers, product variants and refunds from your own backend to Eyk through the Data API or BigQuery hook tables.

Custom data is the order, customer, product variant and refund data you deliver to Eyk from your own backend. You use it when your shop does not run on a platform Eyk connects to directly. It reaches Eyk through one of two transports: the [Data API](/guides/custom-data/data-api) or [BigQuery hook tables](/guides/custom-data/bigquery-hook-tables). Both carry the same objects and follow the same rules.

One example runs through these pages. Noor & Vos, a fictional home goods shop, sends order `NV-10432`. It holds two vases at EUR 20.00 and one tray at EUR 50.00, a EUR 10.00 coupon and EUR 5.00 shipping, all excluding 21% VAT. The customer is `CUST-8842`, the variants are `VASE-BLU` and `TRAY-OAK`.

## Which transport

|           | Data API                                              | BigQuery hook tables                        |
| --------- | ----------------------------------------------------- | ------------------------------------------- |
| You send  | JSON over HTTPS, up to 100 objects per request        | SQL `INSERT` from your own BigQuery project |
| Fits when | Your backend can call an API when a record changes    | Your data already lives in BigQuery         |
| Only here | Cost corrections from a second system, such as an ERP | Rate cards for shipping and payment cost    |

## Rules that apply to both

### Amounts exclude tax

Every amount excludes tax, and `taxes` is the tax on top. When your system stores prices including tax, divide by 1 plus the rate before you send. Noor & Vos stores the vase at EUR 24.20 and sends `unit_price` 20.00. The order fields `subtotal`, `discounts` and `taxes` are the truth for the order. When the lines do not add up to them, Eyk keeps the lines and books the difference on an adjustment line, an extra order line without a product. Send discounts as positive numbers. Refund `amount` is the goods refunded and excludes shipping.

### Ids must match exactly

Eyk links records only through the ids you send, compared as strings, including case. An order line's `variant_id` must equal a product variant's `id`. An order's `customer_id` must equal a customer's `id`, and that customer needs an email for new versus returning customers. A refund's `order_id` must equal an order's `id`. The purchase event of the Eyk tracking pixel links to the order when its `transaction_id` equals the order's `id` or `order_number`. A link that does not match fails silently: the order still counts, but without the product, the customer or the attribution.

### Only products are items

An order item is a line for a product variant in your catalog. Shipping goes in `shipping`, coupons and other discounts in `discounts`. A fee, a service, insurance or a gift is an item with a variant row of its own. An extra parcel is the same order, never a second one. An item whose `variant_id` matches no variant keeps its revenue in every total but has no product in product reports.

### Send a record again to update it

Both transports are append-only. Per `id`, the version with the latest `updated_at` wins, so a re-sent record must carry a later `updated_at` to replace the old one. There is no delete. To reverse an order, send a refund. Eyk does not filter on `payment_status`, so leave out orders that never became a sale. Eyk processes everything once a night, and data shows up in reports the next morning.

### Costs make profit

Profit is net sales minus cost of goods, shipping cost and payment cost. Cost of goods comes from `unit_cost` on the line, or from `fallback_unit_cost` on the variant. Shipping and payment cost come from `shipping_cost` and `payment_cost` on the order. Without those, Eyk uses a rate card per method in the hook tables, or the fallback in your Eyk settings. A second system that knows the real costs, such as an ERP, corrects them through order updates in the Data API.