UCAS reply?

Hey everyone. I've a question about the UCAS confirmation slip. The date on the letter said 30th July, and I was to reply "within 14 days of this letter" (to UCAS) to confirm, otherwise they might give away my place. When I got it I only had a few days, and sent it on the 12th, just before my deadline of the 13th.

Do they count the postmarked date or the date of receival? Am I screwed? It'd be somewhat ironic if I failed at this point...