The officer may have stamped you incorrectly, but It’s your responsibility to check your passport entry stamps. If you had you could have had it corrected free of charge.
The bus takes about 10 minuets to cross the boarder but it’s a dirt road that’s a bit up and down, not sure of actual distance.
It crosses stops by a duty free shop they take your passport get stamps and pay the fee to enter Myanmar, then drive you back. Fee is probably $10 so the rest of the 960.- is for the bus ride.
I did it a couple of months ago. Not a bad trip just 3 and half hours from BKK.