I've tipped for things like this. I've never actually expected them to go out and buy that thing. One time I did ,and she actually sent me a bunch of pictures the next day in New lingerie. I thought that was awesome of her. Lol. Didn't expect thatAre you sure because you seemed to be saying you would only chip in for a coffee - anything more expensive and you wouldn't trust that the model would buy the item advertised and would instead pocket the money?
To be honest (not that anyone's ever bought me more than a "coffee" or "cocktails" from my tip lists) I'd be more likely to actually buy the expensive thing and show them proof afterwards as part of a thank you. The drinks I have never actually purchased, nor been asked to prove it/asked how my drinks were...