The main reason is that it's about 25% of the items purchase price. I believe I can do it for less, about 1/2 of the prepaid cost. Granted, that's a difference of only about $4 but I still have to do almost the same amount of work either way. I pass right by the post office at least twice a day anyway so I don't preceive it as an exceptional convenience.
Certainly not like the difference between prepaid cost and DIY is going to break me but for some reason it just stuck in my craw.