1. In the Custom Fields tab, create a custom user field called for example called "amazontax" or "producttaxcode" (Make sure it is non-vsible and the default option is A_GEN_TAX. (Varchar 50) - For help, check out the guide on How To: Create Custom Fields:
2. In Settings > Channels > Amazon XML Mapping map amazontax / producttaxcode to ProductTaxCode in Amazon.
3. Bulk edit all the guids and amazontax / producttaxcode and leave amazontax / producttaxcode 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.