usually I only end up paying customs every 10 purchases or so.
Yes, Canada Post is very lax about charging tax, duty and their own $10 service fee on imports valued over $20. As you mention, about 10% of your imports have actually been charged. I don't think I have ever been charged in over a decade of buying.
The GPS is run by a private company (Pitney Bowes) and they are much more conciencious about levying these charges than the post office. At least with PB and the Global Shipping Program, the buyer sees and pays the duty and tax (and service charge) before the deliveryman is on her doorstep. Small blessing.
The GPS is still new, but it looks like on items valued under $100, it would usually be best to ask the seller not to use the service, since the costs will be very high. Most sellers do not actually understand how much these costs are. Most sellers will at least try to be helpful and switch to USPS/Canada Post. If they won't, don't buy.
For values over $100, there seems to be a consensus among those who passed Grade Four arithmetic (I 'm dysnumeric and took Latin instead) that the GPS is probably the least expensive importation service, unless the buyer wants to gamble on Canada Post's continuing laxity.
