Below is an issue we are seeing and have a proposed solution . let me know if this is the right forum or should go into Technical Q&A
Problem – When orders are discounted in Shopify checkout, the price that is provided to 3rd parties via the Carrier service API for getting rates is the list price of the items on the cart, the discount item values are not provided. As a result, the calculation of duties and taxes for International shipments is based on the entire list price of items in the order and therefore may not be accurate, leading to situations where the merchant is collecting more in duties and taxes that what the customer should be assessed based on discounted price.
Proposed solution –
Provide a discounted price field at the line item level in the Carrier service API in addition to the list price, so that the 3rd party service can make the determination on the appropriate value to use and return accurate duties and taxes for International shipments.