Next month, if your customer doesn't wait for a combined invoice, you send them as many separate parcels as they paid for you to send.

It might take time, but with luck, eventually people will learn to wait for their invoices if they want postage discounts.

A small positive in this for sellers is that in theory, eventually our customers should largely become trained to wait which will eventually save sellers some time and mucking around in issuing partial refunds etc.

In this instance, you cannot combine the items as you void both the buyers and the sellers protection as two separate tracking numbers must be assigned and a buyer cannot pay for an item in two installments/payments.

You will only be able to assign one tracking number to one of the items, leaving yourself as a seller open to a fraudulent claim of item not received as you will not be able to prove you have sent the second item as you have no tracking number for it.