Great news for you, @Joshua_Winskie - you can do this on Ramp. 
We offer the ability to restrict specific merchants on individual cards.
To do this:
→ select the card you wish to add the merchant restriction to → click ‘Edit Card’ → scroll down to the ‘Advanced Controls’ section. From there → choose which merchants are blocked on a Ramp card.
For your Amazon example, you can use the ‘Block Merchants’ feature and add “Amazon” to prevent Amazon charges from being made on all cards except the primary Amazon card. This way, you can ensure that only the tax-exempt corporate account is used for Amazon purchases.
Let me know if you have any questions!
Charlotte