Ariyapala is not a supermarket, it's a liquor store. A pretty good one. They have perhaps the best selection of wine and a decent selection of alcohol. It's also got the added ambiance of not ordering your drink through literal bars.
Ariyapala is a family run business, the owner is often sitting to the left of the counter, hands and neck bedecked in gold. The cashier seems used to people buy little bottles of arrack and is usually quite nice if your order anything else. There's a wide selection of imported liquors (ranging from Kahlua to Sambuca) but the more reliable sellers are beer and arrack.
They also have an impressive wine selection. As far as scale, Ariyapala is probably the best liquor store in town.
For beer and stuff, pay first, then collect.