Payment for the Product are executed through reseller's checkout Paddle.com. It accepts Credit Cards, PayPal, Apple pay ang Goole Pay payments.
Yes, each subscription plan gives the unlimited domains license. Within one subscription you can also generate the separate license keys, for example if you would like to provide your client with it.
It depends on the country you are from and wheter it is a B2B or B2C sell. Check the article that explains when you will need to pay VAT.