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Common Links allows you to add regularly used links to the search results when linking text content of your post.
Add your common links from the Tools menu. Then, when linking content, begin typing the link title or URL to pull up and insert the link.
Note: Entering <code>.</code> when querying links will not return results. Use spaces instead to match URLs.

Common Links

Adds custom links to the editor's link query

Dan Griffiths

Adds a robust third-party product system to Easy Digital Downloads. When activated, adds new fields to the Download Prices meta box allowing you to specify redirect details.
External Products is based on the extension of the same name by WebDevStudios.

Easy Digital Downloads – External Products

Was looking for exactly this: an option to add third party plugins on my own EDD shop. 

Perfect

Adds a robust third-party product system to Easy Digital Downloads

Common Links vs Easy Digital Downloads – External Products