I found a letter in the mail with a seed that I had entered before and there was another one immediately under it, I tried it and my bitcoins were displayed!
Mailing mnemonic seeds sounds like terrible security, but I'm glad you found it back!If it's a substantial amount of Bitcoin from before August 1, 2017, you'll also own several Forkcoins. Those are currently worth about $500 per Bitcoin you had, so after securing your Bitcoins, you may not want to throw out your seed phrase yet.
Where I can see Forkcoins?) My wallet created in December 2017