1. You can ask the buyer to make the purchases and to wait for an invoice before they pay.

2. The buyer can add the items to the cart and if the listings are set up to combine they can pay immediately.

3. You can ask what items the buyer wants and set up a listing with those items.

If sending an invoice after purchases, simply adjust postage and send. (Do Not click recalculate) 

My preference is for option one.