Credit card authorization nightmare since for years! How do you handle this...

We authorize the customers card when they place an order.  Sometimes the items they ordered are on backorder, or they call us and make changes to the order, so it changes the total amount on their order.

For example, an original $100 authorized order could now be changed to a $85.  We fulfill and bill the order at $85 but the authorization of $100 is showing on their credit card account as being deducted.  I know that it is only an "authorization", or as the credit card companies sometimes call it a "soft charge", but the customer thinks we took $185 from them.  We have to explain to them that the $100 one is only an "authorization" and this will expire in a few days, but some customers get irate and want the funds available to them