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.