Please note that "100" is contained in the "Trade %" Field. This means that BookAccents will examine each line item preceded with a "Credit Allowed" Flag (an asterisk (*)), and apply the trading rule specified in the "Trade %" Field.
To check the math, there are two books totaling $10.88 flagged as "Credits Allowed". The Sub-total of the invoice is $26.37. Since the customer provided credit slips of $33.55, sufficient credit was available to fully cover the $10.88.
The math is ($26.37 - ($10.88*1.00)) + .93 Tax = $16.42. $20.00 was tendered, and $3.58 was the change. You can see these calculations in the fields just below the item list. Also focus your attention on the fields to the right of "Credit Slips". They show that the full amount of $10.88 was used on qualified books during this transaction.