Self-addressed envelope (international)

This is my first time sending mail to the UK and first time sending anything with self-addressed envelopes with paid postage.

How do I find the price list for the amount of a self-addressed envelope to the UK through USPS? I place the self-addressed envelope inside the original envelope I’m sending, right?

Sending fan mail and requesting signed photo back, they’re asking for a self-addressed envelope with paid postage. I’d like to know how much it’ll cost before I go to the post office but there’s no option on the website nor options on the customer support line. I have terrible anxiety and I’d like to be prepared.

Thank you very much!