Well, it looks like FetchApp just creates download links. It doesn't actually handle the order process. And LimeLM really negates any use of FetchApp (because customer will need a product key to activate your app -- "protecting" the download link is somewhat pointless).
Short answer: just host your apps on a cheap webhost and use LimeLM for your licensing.
Longer answer: you can use FetchApp if you want, but it's not really doing anything useful.