They will ask you for 500 Baht per letter, I did 2 last month for licence renewals. Tell them you don't want it today, they will send you back to the front desk and give you a receipt with a date one week later.
Mine was dated Friday, I think 2 pm, I wanted to drive to Pranburi to get my licence that day so I went about 11 am, the lady on the desk tapped my receipt and pointed at the clock, said come back at 2 pm.......they've lost their little bonus so won't be flexible with you.
So if you plan ahead, go a week before you need it and it's free.