How to set Tax Rate / Tax Code on Sales Order Items

Using the PHPToolkit to create Sales Order records, and the tax rate and tax code are automatically assigned to each line item, based on the tax settings assigned to the customer. That is fine, except that the customer's tax rate is applied to *every* line item, ignoring the "isTaxable" setting in the record for each product. The result is that products that are defined as not taxable are still applied tax when an order is created through SuiteTalk.

Is there a method to override the tax rate / tax code for an individual line item on a Sales Order? Or a setting that will get SuiteTalk to respect the "isTaxable" setting on each Item? I have tried to set "taxRate1" via SuiteTalk for individual line items in a Sales Order, but that generates an error that the field is read only or cannot be modified.