After the 6-month period is over you can buy extended support. You can buy extended support in the beginning, too. I that case you get a 1-year support in total.
Yes, SaasLand comes with ThemeForest’s standard 6 months support. We conduct the support via theme’s “Comments” section and also our support ticketing system.
The payment is one-time. After you pay the one-time price of the theme, you will never be charged for it. But you can buy extended support after your support expires after 6 months.