I put wax seals on my thank you cards. I hand cancelled them at the post office, to avoid having them damaged by the machines. The clerk at the post office got a little p!ssy because I had so many. He took a few of them and said that he could put those in the 'cancelled' pile. He told me to leave the rest on the counter...
My friend's dad works at the post office and called the guy to follow up. The guy put all the rest through the machines (even though I hand cancelled them)
So far: the ones that were going to the same city as the post office arrived fine.
A few others arrived fine.
A bunch arrived to various locations around the country, but with the wax seals destroyed. Some were partially damaged, others appeared to have melted into a wax glob. Some people said that they didn't even know they were supposed to be at all...
A bunch have not arrived- and they were not traveling very far...
So, my advice would be to receive confirmation from the post office that they will be careful with your mail, if you are going to use the wax seals...