Many people have posted their experiences here about that. A single letter or number wrongly typed would result on a rejection without refunds. Be careful.
I don't think Immigration would ask (yet they can, of course). The problem is mainly with the airline. Most airlines won't let you board a one-way flight unless you've got a visa.
Participante anónimo 401 in convenience stores the minium amount to use credit card is 200 baht, in some restaurant 500 baht. The QR code comes handy when you need to pay only a water bottle or a pack of mangoes. It can be used also for the BTS and MTR.