Spas often have a commission on retail sales, but I think most MTs are uncomfortable with a retail push. Spa/beauty salons usually aim to get about 50% from retail 50% from treatments, so if someone has a $100 facial you are expected to sell $100 retail to them. It's more difficult with body treatments as it's not quite such a pat face feel how good skin feels must buy product to do this at home feeling, it's I feel so much looser/lighter/taller, the therapist was great rather than the product made me feel great, must buy now often the commission goes up after you've made a target (or you don't get commission until you reach a target)
I don't personally know of anywhere that has an incentive on how many treatments you do (and because of all the reasons moogie303 mentioned) though I think someone on here did mention they had a rebook bonus - that would depend on where you are located, rebook in a tourist area where most of the clients are a one time only tourist visitor would be good, but a rebook bonus where it's a stable client base where everyone rebooks would seem a bit pointless.