thanks, zorpner! I am actually starting to work with artists already, very similar to what you mention. Here is an example:

https://www.levleframes.com/artists/joey-roth/frame?levle_id...

The best part is that no technical integration is required (drop a postcard into the shipping tube with the URL or email it with the receipt).

Thanks for the feedback on mat samples and the acrylic. Some details on the Frames page, but I can be more specific. Museum glass is great but decent risk of breaking in transit, hence the use of the acrylic.