Thanks for posting to the Community, @schalk.
Let me show you how to record the 12 x 300ml stock you have and break this down as a 1 x 300ml item.
First, you can create a purchase order for this stock. Please make sure that the quantity is recorded as 12 so that you can breakdown this item as 1 x 300 ml. Then save this as an inventory item.
Let me show you how:
Once done, go to + New to create a Bill and enter the supplier's name and add the Purchase Order. Then hit Save and close.
Lastly, go to + New and then select Pay bills. Check the box beside the supplier and then click Save and send.
You can now check the item under Products and Services in the Sales menu. You can apply necessary changes like editing the item's sales price rate, reorder point, and the cost of the good.
You can now create an invoice to breakdown the cases received.
You can refer to this article as your reference on how to turn on purchase orders and enter a purchase order: Create purchase orders in QuickBooks Online.Also, you can give customers the flexibility to choose when and how much to pay, check this article: How to create recurring invoice and manage recurring transactions.
Please let me know if you need further help. I'll be around to assist you. Take care!