[lathiat]

Something often not accounted for is these stores offer gift cards at much higher payment fee rates (seemingly at least 10%). And you need to avoid a scam where developers just syphon that through.

I believe at least some of the time they also do currency conversion for you which typically nets another 3%

Of course they could always charge a variable commission based on the funding source etc and also charge less than 30 still (eg Apple Cleary thinks it can do 15%). But their cut is not always as simple as 3% or less.