In 2007 I found a very useful service for people in the UK wishing to send letters. You can buy postage online from Royal Mail, download a stamped and addressed PDF file, print and send.
I’ve used this service a lot as I’ve never got the time to go to the Post Office to post parcels, and can never find my stamps for normal letters. Using the online service you can buy standard postage so long as you know the weight and dimensions. It has been especially useful for sending eBayed items.
I use a prepay account, and last month, they did something curious: my card was about to expire, so they credited my card with the amount I had remaining in my prepay account. For a moment I thought they had taken my money, but it was safely back in my card’s account.