I've had models send me a quick personal "Thank you" message either via email, Snap, Kik, etc. I'm happy with it being just a simple "Thank you". But, sometimes they've done a special picture or snap with it. Other times, I've received a small photoshoot (5-10 pics depending on what it was). Other times, I've seen models do public thank yous either on cam, or on Snap.
Probably the biggest thank you/reward was on an impromptu purchase. I received a snap from a model I was a regular of, she was in a fitting room and wearing a dress she said she really liked. After a couple of messages, and more snaps of her in it, I wound up purchasing it for her and some extra on top of it. Later that night, I get a message from her that she sent me a video. Then, a few days later, I receive a package in the mail which was the panties she wore in the video.
I'd say it all depends on what the items are, and how well you get a long with each of the members who purchase them. I'm more than happy with a simple "Thank you".