A delivery charge for stamps.
I mean, the post office guy comes to my apartment building every day with our without something for me. So, if I want to buy stamps online from the USPS (yes, it's .com; no, .gov doesn't work--another mystery), why are you going to charge me extra for something you're going to do anyway?
Now, if you want to add a handling charge, we can talk. But delivery?
Even UPS gives its customers its mailer label holders out for free on their next time out!
