GST while processing AP Invoices in PO module

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Hello,

I'm looking at processing AP invoices in PO module and the issue I have is around GST. We are in Canada fyi.

We don't include GST on PO as tax and add GST line while entering invoice in AP module. But PO module won't allow entering any new lines. It's just about matching an Invoice to a receipt. 

Does anybody use this functionality? How do you deal with GST?

Thank you,

Sergey