If your organisation is looking to implement IP Restrictions for the Payments2Us User, then following is applicable.

The main Payments2us user that is connecting is the person that Authorises Payments2Us.  This is a Salesforce user.  So, the IP Address you need to restrict are Salesforce Server IP addresses.  Please consult the Salesforce help guides for these IP address ranges.

Please note that Salesforce can change servers for redundancy fail overs or even during upgrades.  You should therefore NOT restrict this to a single IP Address, but all that are listed with Salesforce.  You should also update this list should Salesforce add new IP Ranges.