It'd depend on what you're selling, surely? I've never had any problems of this sort with basic retail product, for example

Well, let's say you sell online, no matter what you're selling, you need to apply the PST/HST of the province the buyer's address is in. And then you need to file a tax return for every province you sell to. Now some provinces have a sales threshold under which you don't need to do a separate tax return, others don't. Some have an HST collected by the Federal government, others don't. And all the PST/HST rates are different. This also applies to digital goods, SaaS, etc...

The US has higher thresholds in every state and no federal sales tax, and buyers pay the duties.