Since AWS built a DC in Canada, I’m paying HST on my Route53 expenses, but not on my S3 charges in non-Canadian DCs.
I’m not an HST registrant (small supplier, or if you’re just using services personally), so there’s nothing to self-assess.
Even if self-assessment was required, you get some deferral on paying (unless you have to remit at time of invoice?).
I believe it works differently in EU (i.e. US DCs taxed) as per Article 44 the place of supply of services is the customer's country if the customer has no establishment in the supplier's country.