Need an advice about creating extension.

[jareq]

Hello i'd like to develop module that will stop customer when he attempt to buy one "Small packet" from the item list and warn him that he needs to order at least 5 of "Small Packet" to make succesfull order. Also i need this to allow client order one of "small packet" when he already has 5 of them.

Small Packet is an item in shop (its vps server).

 

Is it possible to make? Do i need module ? addon ?

I've read WHMCMS Provisioning Module Docs but im still not sure how to do this.

I would greatly appreciate any advice cause im not really sure where to start.