When end user receive promo credit, we generally recommend developers to set CPA to include tax so that end user will not require to pay for tax.
As for shipping fee when end user is eligible for physical rewards, end user will be responsible to pay. If end uses Amazon Prime, they are eligible for 2 days and non Prime member will get free shipping fee if order exceeds $25. Physical rewards will not be shipped from 3rd party seller. Please see general shipping guideline from here. https://www.amazon.com/gp/help/customer/display.html?nodeId=201910610
For detail information on taxes and shipping fee, please check out below.