1.  In the Advanced Tab, Create a Custom user field called "amazontax" (Make sure it is non-vsible and the default option is A_GEN_TAX.  (Varchar 50) - For help, check out the guide on Custom User Fields

2.  In SETTINGS > CHANNELS > AMAZON XML MAPPING map amazontax to ProductTaxCode in Amazon.

3.  Bulk edit all the guids and amazontax and leave amazontax all blank in the spreadsheet. (Because A_GEN_TAX is the default it will send this in)

This will apply to default amazon tax options to all your listings.

From here - you can submit different tax codes as needed for your products.