I went through this process a few months ago and to save any worrying followed the advice and bought the cheapest flight online to a neighbouring country, it cost me about £20.
As James stated the 45 day + 30 day extension seems like the simplest and best solution for you. From personal experience the 30 day extension is very easy to obtain.
You can always book a cheap flight from Thailand to somewhere like Vietnam within 30 days to save you worrying. That's what I did and it cost about £20. I never used the flight.
As James stated, this is normal practice, so nothing to worry about, unless they found something really irregular about your application the next step is a formality. Congratulations!
Hi Trevor, the non OA visa is if you apply from the UK, the non O is if you apply within Thailand for reasons like retirement. If you apply for a tourist visa in the UK this gives you an initial 60 days and you can extend for 30 days easily.