I tried to sign up at USPS a few months ago, because you need an account to order stamps. They didn't let me choose a username or password, altough it fulfilled their requirements (minimum 6 characters etc.). It doesn't matter anyway, because there are so many members HERE who buy me US stamps with no additional charge.
And yes, it's true, Royal mail does not ship stamps outside the UK! But they don't say it on their website. I placed my order, they charged my credit card and after a few days they refunded the money, without any explanation. I emailed them and they said they don't ship outside UK and also the address of the credit card holder (me) has to be a UK address as well!
Weird!