I noticed people mentioning using wax seals with their letters. I have one advice to share abut using wax seals with your letters that you are sending by USPS.

Use your wax seals to seal the letter itself or next to your signature instead of using the wax seal on the envelope. I say this because I receive letters from my penpals who use wax seals on their envelopes and they arrive messed up because letters/mail run through machines that scan and sort letters. It’s a shame that the wax seals get ruined.

I personally add wax seal next to my signature. And I’ve sealed letters with a paper belt and wax seal too. The wax seal is protected inside the envelope and doesn’t get ruined during the sorting and scanning at the USPS.