751—5.13 (8D) Modification or withdrawal of bids or proposals. Vendors may modify or withdraw bids or proposals prior to the time and date set for the bid or proposal opening. Modifications or withdrawals shall be in writing and delivered in a sealed envelope which properly identifies the correct bid or proposal to be modified or withdrawn. Vendors may withdraw a bid or proposal after opening only with the approval of the commission if the commission finds that an honest error was made by the vendor which will cause undue financial hardship to the vendor and which will not cause undue financial hardship or inconvenience to the commission.