UPDATE: it’s now possible to get residence certificate same day, pickup at 5:30pm. Charge is 1000 baht. The old service of picking up next day is still available, pickup at 5:30pm, charge is 500 baht. I presume the “free” one is still available if you want to wait 2 weeks, but I didn’t ask.
You will only get 15 days if you go to Mae Sai or cross any border via land. You need to fly out / back to get 30 days (which can be extended for another 30, for a total of 60 days).