Every Gloriads blog article tagged HST.

Amazon already charges GST/HST on an unregistered seller's amazon.ca sales. Registering costs a Canadian seller nothing there and returns the ad tax.

A Canadian KDP author with no GST/HST number pays 5% to 15% tax on Amazon ads and never gets it back. Registering lets you claim it. Rules, cost, example.
Gloriads runs your Amazon Ads by rules you control, on all four ad types.