Hello there, lunr.
Since you're creating invoices using the API, I want to direct you to the right support that can help you override the tax amount for one line item.
We have a separate online forum called Intuit Developer page where you can post this type of question. I suggest reaching out to them as they can provide insights and guide you through the process of uploading receipts.
You can always go back if you have other questions about QuickBooks. Have a good one and take care always.