"We are in US, you are not suppose to charge us tax".

One way around that may be something I was recently exposed to with a project developed for an international franchiser. Instead of seperating the tax from the purchase amount, they, instead, included the tax with the price. The tax would be based on the catalog (full), or "retail value" of the of the purchase, even though the franchisee would only pay 50% of the retail value.

IOW - there was no seperate line item for the sales tax.

It seems the issue is not your price, but the fact that your invoices have a line item for the sales amount and a seperate line item for the CRA.

Couldn't you just just combine price and tax as a "single" sales amount and then back out the CRA's taste after the sale was closed? I bet the CRA could find a way to allow you to do that. The better your sales the better their receipts.