Insert a second Body section in the report. Create two form letters. Place a letter that encourages customers to buy more in the Body Section A of the report; place a letter that encourages customers to bring their balance down in Body Section B of the report. For more information, see
Creating a form letter using a text object.
Use Condition Formulas to format the Body sections so that each is suppressed under certain conditions.
Now, when a record indicates available credit, the "buy more" letter will print. When the account is over the credit limit, the "over limit" letter will print. And when the customer is right at the credit limit, nothing will print at all.