Interesing idea! It'd be a good idea to have the prices increase because its really easy to just get a bunch of merch stores and then never have to worry about money.
It would be nice is a buy multiplier so that you can buy 10 things at once.
Also maybe consider using long instead of int because I managed to overflow the lifeforms saved into negative values (int owerflows at like 2.5 billion and long at like 9.2 quintillion) or since you dont need negative numbers anyways ulong (overflows at like 18 quintillion) would be even better.
Another suggestion is maybe having the offers stay on screen a bit longer but thats a minor thing.