If you are running your own Amazon account within ATEP (Amazon Tax Exemption Program), unfortunately it is not possible to remove tax exemption for an individual order, even when that order does not legally qualify for tax exemption! It's also impossible to turn off tax exemption on Amazon once you've turned it on. This means a particular Amazon account, if registered under Amazon's ATEP program, will become legally unusable for non-exempt orders.
You can overcome this problem by using Managed Accounts. Managed Accounts take all the hassle out of opening Amazon accounts, receiving tax-exempt status, funding them, and more. If you are dropshipping to eBay or another marketplace, take a look at PriceYak Managed Accounts. If you're hoping to place tax exempt orders for another use case, you should use Zinc API Managed Accounts.
When using Managed Accounts, you can choose to keep your existing Amazon account for placing non-exempt orders. Or you can simply switch off exemption within your Zinc Managed accounts, in which case your orders will automatically route to non-exempt accounts.
Please email firstname.lastname@example.org if you have any questions! We'll be happy to help.