There are obviously quite a number of ways to tackle this, but I would suggest that in you system you need be able to identify the monthly sales created by the facilitator (for example, set the product up as a inventory item so they are going to sell the inventory and at the end of the month they can run a sales report by inventory), so you can work out how much to share with that contractor.
In this case you are in a better position than a seller to determine the price of goods or services. The business will then issue the tax invoice rather than the seller, through “buyer-created tax invoice”.
Buyer-created tax invoices are most commonly used in New Zealand’s primary industries such as farming, fishing and wine-growing.
You can refer to the below link for more info:
Hope this helps.