It's about little desserts that restaurants can charge $2 or so for. You get your coffee and a little creampuffy thing to go with it.
I think it's the wrong tack.
I'd just bring the little thing for free.
Don't announce it, don't make it a big deal. In fact, only bring it for regulars, or irregularly or when people ask for it. Irregular reinforcement is a hugely powerful message sender.
When something is a bonus, it feels great. When it triples the price of your coffee, you want it to be better than it actually is.