Yes having to scan you bill/receipt is the approach I am thinking of but how are providers/supermarkets willing to give this data? or I have to find away to get these information somehow.

As I wrote in another comment, I would like to know how many are interested on this idea to see if the journey is worth to take.

@trtobe: you could ask the supermarkets. Doesn't hurt. Perhaps you would have better luck promoting this idea with grocery stores that have loyalty cards? Loyalty cards I suppose are already used to track specific users and what they purchase. I think working with the supermarkets are better because it results in a more frictionless experience.