I have already written about surcharges relating to fake stamps but the next most common cause of a surcharge is using an A4 envelope and sticking a standard stamp on it. A standard letter size, above, like your bank statement or council tax bill arrives in, takes a standard stamp. Anything larger than this needs a large letter stamp. If you put anything in the envelope with your letter or birthday card it will need a large letter stamp. Birthday cards with badges on will need a large letter stamp. The thickness limit on a standard letter is 5mm, this is not very much. Just because you are using a small envelope does not mean it is a standard letter.
The revenue protection people do their jobs pretty enthusiastically.
£1.50 is the standard fee for underpaid letters.
(Disclaimer: this A to Z is not official Royal Mail advice, except by coincidence.)
Stay safe. Be kind. Don't get yellow stickered.