I was in a US post office for the very first time today. Something I noticed was a sign on the wall saying "Your letter must have a return address." The word "must" was underlined.
Now, I've always found it slightly odd that folks here in the US like to write their address in the top left corner on the front of envelopes when they post them. I don't much like the convention that you are *expected* to do it: I like privacy. But it doesn't bother me hugely. I still do it. But I always thought it was optional. And the internet thinks it is. But not this post office: that word "must" was underlined. How odd.
I should have asked about it!
Instead I just posted a letter, bought some UK stamps (for 98c each) and some US stamps ... they must've been about 50c each, maybe a little less. (I don't have the receipt handy.)
They call them "forever" stamps: they're first class stamps that don't have the price on them, so they don't "expire". We've had them in the UK for, like, forever.