If you're anywhere near the border (I'm not far from Toronto), it may be worthwhile to get a US post office box. We've got one, as my husband lives & works in Austin, and this way I can bring anything I purchase back over the border myself, and show Customs the actual receipt and product, and they tell me exactly how much I need to pay - not the guestimating that the mail offices use. (The UPS Store in Lewiston will even receive one-off packages for a $10 fee, without having to have a box, you just have to contact them first.)
UPS is by far the worst when it comes to cross border fees. Fedex is marginally better. USPS is best, but still not great (The others use a broker which "estimates." Yeah. If you buy less than $20, there shouldn't be any custom fees if that value is marked on the waybill. (DO NOT try to get companies to declare that whatever you purchased is less than $20 if it's not. They get in serious trouble, and so do you - we used to have customers try to get us to do that all the time.)