Buyer paid for two listings before I sent invoice as I was waiting for another listing to finish.

They paid by PP but I want to send as a combined parcel,(3kg C & S seems the best option) as they weigh over 500 grams combined,(will only charge them the $3 + $3 = $6 as per listing).

Forgot to add the registered to those two so I'll cover the rest.,($4)

The other buyer didn't win so the other listing will be going by registered letter rate,(saving the $4 there).

So how can I combine those two sold items into one package please.