Coffer Key Glue
Sold by Naleidea Rivergleam. This does indeed take your Coffer Key Shards (in quantities of 100) and convert them into a Restored Coffer Key.
Purchase 
Coffer Key Glue from Naleidea Rivergleam in Silvermoon City to convert Coffer Key Shards into Restored Coffer Keys anywhere without visiting a vendor.
Guide awaiting review
Locate the vendor
Find Naleidea Rivergleam at 52.6, 78.0 in Silvermoon City inside the delve building.
Purchase the toy
Buy Coffer Key Glue from Naleidea Rivergleam using Undercoin.
Tips
You can convert Coffer Key Shards to Restored Coffer Keys for free by talking to Naleidea Rivergleam or by entering a delve automatically—the toy is optional.
You are capped at 600 Coffer Key Shards per week; converting shards does not let you exceed the weekly cap on Restored Coffer Keys earned (maximum 6 per week).
