Programmatically edit an existing Shopify order to add a line item using the orderEditBegin / orderEditAddVariant / orderEditCommit flow

In-depth guide

Steps

1. Call `orderEditBegin(id: $orderId)` to open an edit session; this returns a `CalculatedOrder` object with a `calculatedOrder.id` that you use for all subsequent edit mutations in this session
2. Use `orderEditAddVariant(id: $calculatedOrderId, variantId: $variantId, quantity: $qty)` to add a new line item to the calculated order; the response includes the updated `CalculatedOrder` with the new line and revised totals
3. Optionally apply a discount to the new line with `orderEditAddLineItemDiscount(id: $calculatedOrderId, lineItemId: $lineItemId, discount: {...})` before committing
4. Review the `CalculatedOrder.totalPriceSet` and `taxLines` to verify the order total and tax impact of the edit before committing
5. Call `orderEditCommit(id: $calculatedOrderId, notifyCustomer: true, staffNote: "Added item per customer request")` to finalize the edit; this creates a new version of the order and optionally emails the customer
6. After commit, query the order with `order(id: $orderId)` to confirm the line was added and that `Order.currentTotalPriceSet` reflects the edit