Working and secure crypto is based on the fact that you get different ciphers for the same massages.
So if you run „21“ through you encryption you get „xy“ as your first cipher. If another user puts in „21“ and runs it through encryption the output should be now something like „ae“. That’s not the case with FTH you always get the same result. That’s bullshit. Because if data leaks you can reverse engineer the ciphers without having a private key. If your encrypted data can be decrypted without the private key. Your encryption ist just nonsense.
tldr: it cheap to make you own prints, google it and go for it!
Imho:
Make your own graphic designs on your computer
Look for nice organic and/or fair produced Shirts (a small quantity should be enough to start with, dont you think?)
Than buy yourself a diy Kit für Printing Shirts