There are two simple solutions to allow combined shipping

1) Allow sellers to cancel labels immediately and refund either buyer or
seller immediately - and then open up Packlink to select the correct
postage. The same solution works for parcels of different size for which
the label is pre-prepared for, allow buyers to opt to collect once they
realise they are nearby, allow sellers to deliver for free, and allows
purchase of expedited shipping. In fact just allowing the labels to be
cancelled would solve most problems. It is easily done by asking sellers
to confirm the label is correct before issuing it.

2) Allow sellers to opt out totally

there is a 3rd way - scrap the whole thing