Seems like a fun idea, but sadly the bot doesn't like my images (tried with 2 images): https://i.imgur.com/848OFut.png

Also, at least in Germany, I can use Deutsche Post/DHL to do the same thing, for cheaper: https://www.deutschepost.de/de/f/funcard.html

I used to work for UK startup https://www.touchnote.com/ - which is a similar idea. I like how simple your interface is. And bot interaction is very hot right now. You should contact them to see if they're interested in it.