Amazon’s tax engine treats “mixed bundles” (taxable + exempt items sold as one SKU) by defaulting to the most taxable component in the bundle, not by prorating tax per item. In practice, when you assign a product tax code to a bundle, Amazon ignores the internal mix and taxes the whole bundle based on that single code.
So if a bundle contains one fully taxable item (like a toy) and one exempt grocery item (like crackers), and you code the bundle as general merchandise (for example, similar to 500000), the entire bundle is treated as taxable in states where that rule applies. If you want accurate handling, you typically must either:
Split the bundle into separate SKUs/lines, or
Use a bundle-specific tax code that correctly reflects how your target states treat mixed kits, then verify results via test orders or the tax calculation preview.