You can do this, but be aware you will only have one tracking number for 5 items, so potentially leaving yourself open to a case for some of the items not received, and you would have nothing to prove otherwise.
If you wish to combine postage, go right of the item sold to More Actions, and select Send payment Details, amend the postage there to that required, but do not click on reset. Check it is correct, and send .
If you are on the Seller Hub, you click on the item in Orders >> Awaiting Payment, then top left you will see send Invoice, follow the above instructions.
You may need a laptop / Pc to do this, and not the app or mobile browser.
@terryhorrobin