JIT and EOQ sum, Penalty Calculation

This is a sum from the BPP Revision Kit, Q6, Q page no.8, answer page no. 74.

Here since the customer wil have to penalty for failing to make the payment within the allocated time he will have to make a penalty payment, so in the solution why has this penalty payment been deducted. It is an income to the company, right ? and hence should be added.