Offhand, I can think of two reasons your registration script might not accept our IPN-style Integration submissions: either it's trying/expecting to validate the IPN with PayPal (which would fail since we are not PayPal, and we already validate the original IPN with PayPal in the first place anyway), or it may be getting confused by the extra fields we add to the basic IPN data set, to pass certain order data we collect that PayPal does not.
I would suggest sending my speculations above to the Aquatic Prime tech support staff, along with a link to our Integration page, for them to consider (or just send them a link to this forum thread). They might be able to provide you with a modified PHP script, or give instructions for modifying it yourself, to workaround the issues I described or any other issues they may identify about using our Integration with their script.