You as a buyer cannot change any detail on the original listing, and only leave feedback for the seller if you wis.
The seller also cannot change any detail once an item has been purchased and paid for. You can send them a message to explain if an item was originally listed with only a postage option, then collected, there is no collection code to give to the seller and the item remains ' not dispatched'.
This can result in a delay for the payout to be released, but nothing you or the seller can do to speed that up.
@alyg84alexandra