I tried setting cost to -1 scrap but it still desont work in game, not sure why? 

Did you try granting the permissions in the documentation?

• vehiclevendoroptions.free.scraptransport
• vehiclevendoroptions.free.minicopter

I did, It says free in the chat dialog when speaking to airworld, but when you click purchase it say you do not have enough scrap.  I only changed the config, i did not grant the perm free mini. I'll try that now and get back to this thread. 

Last I checked, the free permission was allowing purchase. The plugin accomplishes this by doing 2 things. First, when the player is about to be shown the option to pay, the plugin sends a fake inventory snapshot to the player which tells their game client that it has enough scrap. That is necessary because otherwise the client would simply not show the button to pay. Second, the plugin detects when the player selects the option to pay, and overrides the logic to charge nothing. I'm not sure which of those is not working based on your description, but I'll test it this week to see if I find any issues.

Merged post

I just tested and found no issues.

• After granting the vehiclevendoroptions.free.allvehicles permission, I successfully purchased a minicopter and scrap helicopter 0 scrap, and with 11 scrap (random number).
• After granting the vehiclevendoroptions.allow.minicopter permission, I successfully purchased a minicopter with 0 scrap, as well as with 11 scrap.

I also tested some other cases for good measure, and found no issues.

• After creating a minicopter price config for -1 using { "ItemShortName": "scrap", "Amount": -1 }, then granted the permission vehiclevendoroptions.price.minicopter.-1, I successfully purchased a minicopter with 0 scrap, as well as with 11 scrap.
• After creating a minicopter price config for 0 using { "ItemShortName": "scrap", "Amount": 0 }, then granted the permission vehiclevendoroptions.price.minicopter.free (for some reason, I made the plugin generate the permission name as "free" in this case), I successfully purchased a minicopter with 0 scrap, as well as with 11 scrap.

you the man, followed exactly as you said and got it, thank you!