Incorrect total for transaction requests

When posting a transaction request to Taxjar API, the "amount" is calculated incorrectly.

The value posted by this module includes the total of line items + shipping + sales tax, but should not include sales tax.

Doc: https://support.taxjar.com/article/1063-taxjar-api-how-to-address-bad-math

This results in every taxable transaction throwing an error, and the transaction failing to be recorded in Taxjar.
The error looks like this in dblog:

@"status":400,"error":"Bad Request","detail":"amount must be equal to the sum of line items and shipping, excluding sales tax."