Short answer
HubSpot's KB: automated sales tax needs Revenue Hub Professional or Enterprise. With HubSpot payments, your business must be in the US or Canada. Rates use the customer's billing address; with Stripe, set a tax election in Stripe for local taxes. With it on, each line item needs a unique name. Discount codes don't work on links with automated tax.
1. Payments
Payment setup in my demo portal.

2. Discounts
See discount codes.