The program is not calculating hold credits. When you click on [Edit Customer] and select the [Hold] tab the number of vacation days for the current billing period is correct, but the credit amount does not appear, why not?

Remember for each rate code you create, you have to check each delivery day and also enter an amount to charge or credit for each delivery whether the rate is calculated day by day or on a flat rate for the entire billing period.
 
Review your setup of the rate codes, Tools | Files | Rate Codes. Be sure you have entered an amount to charge or credit for each delivery day. ProCarrier cannot calculate a hold credit unless you enter a credit amount.