On the cleaning solution you have listed?
You cannot change the postage or add a reserve now as you have bids, and the sale has to carry on in auction style to the end with no changes.
The golden rule for auctions, is to always list them for the lowest amount you are happy to receive, as it cannot sell for less. Reserves are expensive, costing 4% of your set reserve, and payable sale or no sale.
@rs2194