I have recently discovered an app called shopmium. They display offers and discounts for purchases. For example a free bar of Cadbury, first you pay full price then scan the barcode and recipient and the cashback is then added to your PayPal account usually with 5-7 working days.
I think this could work well with Monzo, either doing your own cashback offers in a similar manner to shopmium or work with shopmium to introduce it to Monzo customers
Monzo are developing a spending rewards & offers scheme called Monzo Points. May not be quite the same as shopmium, but you might be interested in taking a look if you’re not already aware of it.
I have only started using it this weekend. It’s easy to use and claim back the cashback you just take a photo of the receipt and scan the barcode of the item. Haven’t received the payment yet but it was approved within a few minutes and is processing.
I’ve been using shopmium for almost a year and it’s good for the frequent “freebies” (full cash back) such as a fajita or rice kit, sunscreen, a soft drink etc.
The issues is when you start buying things you wouldn’t normally buy just for the cash back