Keyshop

Used it for a few years now
Does exactly what's needed of it re delivery of digital serials/keys.

Simple to use interface.
Support is prompt and second to none - best support times I've every experienced with no fuss and just prompt resolution.

Can't praise the app and developer enough.

Keyshop has been great in helping me streamline my product distribution. I utilise Itch.io to distribute my products and had previously had to rely the shopify digital attachment app that was severely limited in the way I could organise files and was extremely slow with uploads and downloads. Keyshop has, so far, reliably distributed keys generated by Itch.io to be sent to customers after purchase. Every so often an email might bounce but that has been few and far between and seems to have more to do with the email provider of the customer. Fortunately, Keyshop will always assign a key to an order so should an email fail it's as easy as copying and pasting the key from the order screen when replying to a customer. Super fast, easy to use, and well worth the price.