Any Standard API available to calculate TAX?

Hi friends,

I' am working on a R12 upgradation project. We have an interface program which imports AR invoice information from feeder system using Auto Invoice in 11i. The standard functionality of the Autoinvoice program will break onto separate invoices those invoice lines where the Ship To customer and/or Ship To address are different from one line to another. But we want to have all the taxable invoices with different adress locations to be grouped into single invoice. To overcome this we calculate  taxes prior to being inserted in our custom interface program and insert into the interface tables on a line-by-line