WHMCS API Access
Setup
General Settings
Other
API IP Access Restriction

PayPal
Go to Profle, Payment Receiving Preferences:
Block accidental payment
Change radio to no.

Why? The solution creates itemized invoice through WHMCS. Paying from WHMCS to PayPal, PayPal thinks each item "could" be a separate order. So, you'll get a duplicate invoice ID error.

This is WHMCS or PayPal workaround, it's not PeakAPI.