Combining postage means sending together in one parcel.
It doesn't necessarily mean giving a discount as many buyers presume.
So it is up to you if you can and will give postage discounts on combining items.
I usually instruct buyers to request total and wait for an invoice after they have completed their purchases.
Select send invoice to the right of item in your sold list then adjust postage in the invoice but DO NOT click recalculate, just preview to check it and send.