The reason I was told that many people send via UPS (or other couriers) is that they provide tracking whereas USPS doesn't. I am in Canada so I can't say definitely that this is so but I wouldn't doubt it as I think much is the same up here.
If you are sending a matted print to a customer I would also suggest that the better packaging would be a box versus an envelope but you may be okay if you plaster "Photographs - Do not bend" all over whatever way you go.
Also something to consider is insurance if by postal service. An angry customer complaining of mishandling is not going to be very pleased and the insurance should cover any reprint/remat/resending of the article. You will have to take that into account with your pricing.
"Beauty is more than skin deep"