yes combining shipping has to be same buyer/same seller.
Usually, the seller will put into listing, if they will do that.
The seller has to do it NOT the buyer.))
You as a buyer have to wait until you are done shopping with this same seller and
WAIT for a COMBINED invoice from the seller before you pay.
You have to make sure you read all of the listing carefully.
Combined shipping charge varies from seller to seller.
Remember some sellers do not combine.