FetchApp allows you to specify license keys / serial numbers for each product. When an order is placed, FetchApp automatically assigns the next unused one and emails it to the shopper. This is handy if you sell software or PDFs that have unlock codes.
To get started, prepare a list of your keys separated by commas or one per line.
1) Go to the Product in FetchApp
2) Click on "Edit License Keys"
3) Click on "Add License Keys"
4) Copy and Paste in your keys and click Add License Keys
That's it! FetchApp will automatically choose an unused key the next time an order comes in.