No problem as such. I was told 7-11 had some sort of poster where you could attach them for other people to collect. Haven't been able to spot that yet :)
You can also look at something like booking.com. There are plenty options where you can cancel before any transaction actually occurs. I'm sure you can just show a week or two booked. Perhaps you can email the embassy and find out.
Were you stamped in for 30 days? I'm pretty sure you can go to immigration and get an extension for 1900 baht. But try early as they add the days on top of your admitted until date.