Integration

Hosted payment page or normal integration: HTTP POST/GET with all required parameters, either to default payment channel page or to specific channel page, merchant can allow or disallow the buyer to change payment method (PDF)
Seamless Integration: popup page or lightbox on merchant checkout flow, as simple as modifying the JavaScript snippet that could be found on GitHub
Mobile xdk: support variety of mobile development platforms and available on GitHub
Direct Server API: server-to-server request that allows merchant to handle all the UI/UX with higher flexibility and controllability (PDF)
Recurring API: server-to-server request that allows merchant initiated transaction (MIT) to debit buyer account anytime (PDF)
Offline Payment API: For in-store e-wallet payment acceptance on POS/terminal (PDF)
ISO Message Interface: For card acceptance terminal vendor (restricted PDF)

3 ENDPOINTS for payment response notification (For integration 1-6 only)

Return URL: realtime web browser or frontend direction endpoint for hosted page, seamless integration, and shopping cart module
Notification URL (webhook): real-time server-to-server or backend endpoint for all kind of integrations
Callback URL (webhook): defer update or callback endpoint on non-real time payment such as Razer Cash

IPN

Frontend IPN, applicable to return URL endpoint only: using JavaScript, you may copy from the snippet code from your merchant portal, note that treq=0
Backend IPN for return URL and Notification URL: post back all parameters with additional parameter treq=1 to returnipn.php
IPN for callback URL: just echo "CBTOKEN:MPSTATOK", without the quote(")