It recently came to my attention that simply putting a first class stamp on an A4 envelope is insufficient postage and is something the Post Office worker should have known when he sold me a first class stamp even while I had the A4 envelope in my hand! I don't expect the recipient would be likely to pay the difference which means I'll have to send another A4 envelope only this time I'll make sure that the postage is correct!

Royal Mail have a Price Finder which will enable me to see exactly how many stamps I'll need to put on the envelope, the only drawback is that the amount I need to pay to post the envelope is too low to buy  online. I don't have any Avery labels to print the postage on anyway!