I'm pretty sure you can mail post cards and letters there stamped with the Hogsmeade exclusive emblem.
During our Preview the day before GO the post cards were sold in the Owlery/Dervish and Banges shop and it was hard pressed to get the post mark stamp due to the lines purchasing from Ollivanders, Dervish and Banges and the Owlery as there was the One Owlery register and the One inside Dervish and Bangs.
On the day of GO we saw an improvement, there were Two registers in Dervish and Bangs and they moved a table to sell items specifically for sending mail out to the Owlery where the tables were along with a register and an Adorable post box to slip your post cards in. At that time the limit of what you could purchase to write and send mail in the outside table were only post cards, mailing seals, a few pins and one veriety of stamps (Ministry of Magic).
I had asked to have the post cards I purchased stamped before I wrote them out but was told I would need to write them out first. BIG mistake. The stamp covered most of what I wrote at the top because I didn't know how much room to leave for the post mark. Really LOVELY postmark by the way! After my eldest daughter purchased her post cards she showed me so I did manage to make the last 2 of mine in a fashion where my writting wouldn't interfer with the post mark.
I would recommend that you have your post (which ever kind) post marked THEN write out the post card or envelope as it is not in the same place or the same size as the regular postmark.
I sent post cards to the children of my friends and wrote them as if they were from Harry Potter, Hermione, Ron or one of the Professors. The children who received them were Estatic!! "Mom" or "Grandma"!! "I got a Letter from Hogwarts!!! Harry Potter said for me to keep reading to you and to come visit Hogsmead when we can get away!!" I was really disapointed they did not have the stationary like the Owl Posts that Harry received in the movies but I did buy the seals and wax sets. I thought the kids would be so excited to receive a letter like that. I had no idea they would react the same way with a Post Card! For now, since stocked items are an issue I would either purchase the stationary at the UO on line store or from your local craft/stationary store just incase. I would have LOVED to send my own children (who were actually there with me) a letter Just Like Harry received! Actually, with the stocking of merchandise issue I would purchase the sealing wax with the house symbols prior to going to the park just to make sure you have before your trip, if you have the time.
Do NOT under-estimate the Non-Potter fans attaction to the Post Mark and Stamps. The weekend of GO was Father's Day weekend and my Dad had never watched the movies or read the books. When I told him I didn't know how long it would take his cards to reach him from Florida the first thing he said was "Did you get it post marked at the Owlery?!" He was VERY excited about this! At that moment I couldn't believe it never occured to me to mail it from the Owler. I put reguarl stamps on the cards and put it in the regular mail box at the front of IOA. It never occured to me that he knew about the Owlery, the post marks, the stamps or that he would be so thrilled about it!! I put his cards in the IOA mail box first thing that morning, there was no getting them back.