To offer users a simple way of saving and getting a little in return, what you could allow to do is allow your users to choose the round up of our purchases, e.g. if cost is 23.50, you could offer us to round up to the nearest 5 pound, so i will get charged 25. Then end of the month/year/chosen criteria, you could return that saving to us as a sum amount…?
Just to keep everything linked together, a similar version of this feature has been discussed here -
although the assumption in that post is that you’re rounding up spend to the nearest pound, not setting a different figure to round to like £5.
Lloyds Bank and TSB do this under the name of ‘Save the Change’. The money is put into a savings account.