Macbook Air 2012 memory upgrade I've won a 4gb RAM Macbook Air yesterday. 
I know that the RAM is soldered to the motherboard, so therefore non user upgradeable. 

I was wondering whether my nearest Apple Store could perform an upgrade to 8gb RAM on site? Do they offer that kind of service?
Congratulations, that is a nice price! Unfortunately the RAM is soldered to the logic board. So changing simple RAM DIMM's is not possible. The only way to upgrade your RAM is to swap the old logic board with a 8GB memory logic board. In most larger (official) Apple Stores there is the knowhow on how to swap logic boards. But I do not know the costs and I think it is not an economy wise decision.