Brigada readers are full of resources to share!  Recently Stephanie commented on the “Snail Mail” post located at https://brigada.org/2022/12/11_37259:

I am a huge fan of the Felt app! For $6 a month, you can send three multi-panel cards that are beautiful and customizable and have tons of ready-made really cute designs. I make them right from my phone in just a few minutes. You can easily upload your photos to them and use your own handwriting or choose between many fonts. And the $6 includes the envelope and postage! https://www.feltapp.com/

Others commented that they use the following:

Thanks, everyone!