Where are they trying to buy it from?
If they are only doing a single return for each year in TurboTax Online, then they don't have to pay until they file or print their return. They will get a prompt that will ask for credit card payment, and US credit cards will work (Visa, Master Card, Discover, or American Express).
If they need to do multiple returns for each year, then we no longer offer any multi-return packages for 2019 or 2020 ourselves. They may still find an online retailer who still has these available, but that retail may not sell to people outside of Canada.
