Hi Nigel, it’s not easy. I know people who have bought slightly larger than normal cards only to find that a first or second class stamp just isn’t enough and the recipient has to pay a £1.50 fee to get the card!

The simplest way is to use this easy to use (but tucked away on their website) price finder from Royal Mail https://www.royalmail.com/price-finder. Of course, you need scales to weigh the letter beforehand but once you know this then it’s relatively straightforward.