The Royal Mail Scheme for Franking Letters and Parcels 2008 states in section 7.1 that you must have the correct prices loaded in your equipment at all times. When a Royal Mail price change occurs you must update your franking machine with the new tariff download. Without the correct tariff you could potentially under frank your mail and become liable to surcharges, or over frank your mail and pay too much for your postage.
Failure to use the correct Tariff may render the franking equipment inoperable. Royal Mail may instruct Franking Direct on Royal Mail's behalf to deduct the value of any Underpayment and/or Service Surcharge, which is payable by the User, from the Franking Account which the User has with Franking Direct.
Click here for full details of the Royal Mail Scheme for Franking Letters and Parcels.