Offer to top up another $10-20 (10-20x their cashback) before their next visit and offer them an extra drink, french fries or anything else that's a high margin.
So why not make it more interesting?
From client's perspective, they get:
20$ in prepaid balance
10% cashback from initial purchase
Free drink, fries or other perks
"Wow, now that's a great business that cares about their clients!" ❤️
And the best thing is, you actually do care about your clients
From your business's perspective, you get:
You actually keep the money in your business, and book it as an advance payment, so you don't have to pay taxes on it, until it's actually spent (more here)
A loyal client
10-20x ROI right of the bat, -and an opportunity to create an infinite loop of:
Serve - Cashback - Top up - Repeat
When cashback NOT is claimed
The money is still for you to spend and you don't even have to pay taxes on them, how?
Bueno books every cashback as an advance payment and issues bonuses for clients to claim. That means, until client spends the cashback your business doesn't actually book a taxable profit, while having the opportunity to use that money freely.