Calculate Price Flag got changed from 'FREEZE PRICE' to 'CALCULATE PRICE' when I changed ship to add

Hi Folks,

We have a typical issue on our instance due to which we had a lot of Price issues with Customer. Actually for few of the lines we Freezed the Price as per the agrrement with Customer. But suddenly before a day or two customer demanded to change the Ship To address for shipment. So we changed the Ship To address and created the Invoice. At the time of Invoicing we realized the difference in Price which was aggreed upon.

How to prevent this..?? Is this a standard Functionality..?? If Yes from where this set up can be controlled.