When you make a payment to the other parent, PayPal is one option for making and tracking the payment. Your PayPal account is linked to SupportPay. The other parent needs to have their PayPal account linked to SupportPay as well. To link the account, make sure they have entered their PayPal email address under Account Management --> Payment Methods --> PayPal.
Each parent does not directly see the paypal email address of the other parent so there are no security risks of a direct connection. Once both PayPal accounts are linked to SupportPay money can be exchanged between parents using PayPal.
In addition, you can track manual payments of cash, check or charge by entering the amount and attaching document proof.