I want to add a cost item.
Player right click item, will run the command and remove the item.

Sounds like a great idea and it will be easy to add. Ill try to get it out for the next update.
(My bad didn't mean to close the issue)

Hope to update as soon as possible.

Sorry, this will be coming out very soon. I have ran into a few problems with life lol. The house next door burnt down and we lost internet for a while there, as well as i'm prepping for college so I haven't had that much free time but I am pushing for a release in the next couple of days.

Well, thank you. Don't need to say sorry...

This has been officially implemented for v3.5. I just have a few things to finish up for the release.

This has been added as of ItemJoin v3.5 and it has been released!

You can grab it here; https://www.spigotmc.org/resources/itemjoin.12661/download?version=107086