I use Auction Wizard 2000 and it has let me set up 4 different tax profiles to cover the different situations - no tax, GST, GST & PST, and HST. I have to choose which profile my customer fits into, but the program will calculate the taxes from there (no tax is the default). It will also remember repeat taxable customers, but not always the correct level so I do have to remember to correct that setting.
My biggest problem tax-wise is the people who have their spam filters set on high so they don't get my EOA notices -- when they try to go through eBay's checkout, they often try to weasel out of taxes! I've only had one who was quite indignant that I would dare try to charge GST but, as mahsauction observed, all it takes is an email quoting your GST reg. number. In fact, I've added the number to my EOA template for Cdn buyers - no more arguments ...{{{looking around for wood to touch!!}}}
Glenda
Glenda