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I am testing GSP in sandbox and getting incorrect Subtotal and Total amount. Why ?
The reason you are experiencing this issue is because you have listed items with title starting with "GSP Shipping". In Sandbox if you use the item title starting with "GSP Shipping" then there will be mock responses returned as with specifications in the GSP playbook for Integration.
Please note that there is no other way to test this functionality in sandbox. You will only get mock responses in the response.
Please refer to GSP playbook for Integration - https://ebaydts.com/eBayKBDetails?KBid=2105