How is the tax rate determined when I sell an online course to customers across different provinces?

For a supply sold to customers across different provinces, the GST/HST place-of-supply rules generally require looking at each customer individually, based on information like their billing or shipping address obtained in the ordinary course of the sale, rather than applying one single rate to every sale regardless of where the buyer is. That means an online course seller can end up correctly charging different GST/HST rates to different customers on the very same product, depending on each buyer's own province.

This is different from how many sellers instinctively think about pricing, as one flat price with one flat tax rate, and getting it wrong by applying a single province's rate to every customer can mean under- or over-charging tax across your customer base, with the seller responsible for the shortfall if the wrong, lower, rate was used. The exact address information needed to determine each customer's province, and what counts as sufficient documentation, matters for defending the rate charged if it's ever questioned.

Because online sales at any real volume can involve customers in every province, and the tax treatment of digital products has its own specific rules layered on top of general place-of-supply principles, set up your billing system and this analysis with a tax advisor before scaling sales, not after.