I saw this on one of my receipts and thought it interesting. Most Britishisms make sense if you have a good vocabulary. They are just using words we don’t use anymore, or use them in a different context.
This is an example, but you have to think about it a little. We all know that people who buy stuff from a shop are “customers”. But what is a customer? Well when we put ‘er’ on the end of somethings that means a person who does something. In this case they’d be a person who does ‘custom’. We don’t use the word ‘custom’ as a noun anymore, but you can see what it means.