If you are a buyer and you have purchased multiple items from the same seller you will see that in the "To Pay" section the items are already grouped together for you by seller. This allows you to see all the items that you have purchased from the same seller. There is a button on the bottom right of this group marked "pay or request invoice".

By selecting this process you can do one of the following.

Make an online payment.
Inform the seller that you are paying via other offline means
Inform the seller that you wish to collect the items
Request a combined invoice from the seller.

This request or confirmation is then sent directly to the seller's control panel and an email is also sent to them.