You can give you some coins to your promoters in order to buy items from the shop. Promoter coins are a kind of special coin and they are counted separately from regular coins.
You can assign promoter coins (PC) only from the dashboard.
For each item you add in the item shop you can also set a price in PC other then in normal coins.
Making promoter items untradable
From the item shop settings you can also configure that items bougth with PC are not tradable, but this part requires some changes in your file servers. The C++ part is not provided and you should ask your dev to implement this feature if you want it!
You must add a new column to item_award table and call id sealbind.
If you decide to have items bought with PC untradable, every time a new item is purchased through the shop with PC, the value of sealbind column will be -1.
You should manage this value in your file server source in order to make this item untradable.
My suggestion is to add the same sealbind column to item table and copy the value from item_award to item table every time an item is taken from the in-game item shop safebox.