You can use this API to acknowledge an orderlines, including all of its quantities. Partial acknowledgement is not allowed, all quantities of an orderline has to be acknowledged.
Walmart requires a seller to acknowledge orders within four hours of receipt of the order, except in extenuating circumstances.

The response to a successful call contains the acknowledged order.

In general, only orders that are in a Created state should be acknowledged. As a good practice, acknowledge your orders to avoid underselling. Orders with line items that are shipped or canceled should not be re-acknowledged.