There's several ways to do it:
-Have a dialogue option show up if you have the item in your inventory, without bringing players to the shop screen
-Create a conditional branch. "If Item (the unlock item)", and in that branch, add the special item. In the other branch, have shop processing NOT involve that item.
-Use a plugin, like the one Frostorm suggested.
Personally, if this is the ONLY special shop option you're using, I'd avoid using the plugin. But if you're doing multiple fancy things with your shops, the plugin is ideal because it streamlines things.
Especially in MV, you can do most things plugins can do without them. There's very few occasions when you absolutely NEED a plugin - you're doing a lot of things that would be streamlined with a plugin, the functionality just DOES NOT exist and cannot be 'faked' in MV itself, etc.
I have only recently learned this myself.