To manage print credits so there is a true credit on the account here is what we suggest.
When creating the order, select the package which includes the print credit from your product list and enter the amount of the package.
Once all payments on the order have been received, discount the package for the amount of the print credit, this will leave a negative balance on the order, indicating a credit.
When the clients purchase new items, add them to the Ordered Items tab and the credit will balance out.
SuccessWare will move the money in the background to keep track of the sales tax. A print credit is not a taxable item until products are added to the order.