I do have a suggestion for your app though:
Have it compare your basket of goods across different markets in your region to show you the cheapest option.
I'm pretty sure this possibility is actually one of the reasons they locked down the API.
I've used Data from REWE in the past and made a comparison between a couple of cities in Germany (I believe it was Frankfurt, cologne, Berlin, Munich and Hamburg). Hamburg was by far the most expensive, often as much as 10-20% more expensive.
Also there already exists this reverse engineered project: https://github.com/ByteSizedMarius/rewerse-engineering/
I do have a suggestion for your app though: Have it compare your basket of goods across different markets in your region to show you the cheapest option. I'm pretty sure this possibility is actually one of the reasons they locked down the API.
I've used Data from REWE in the past and made a comparison between a couple of cities in Germany (I believe it was Frankfurt, cologne, Berlin, Munich and Hamburg). Hamburg was by far the most expensive, often as much as 10-20% more expensive.