By using one of the below methods, merchant can get integrated with PayU:
Non-Seamless Integration(Using PayU SDK's UI) - In this mode the customer would be using PayU SDK's UI and flows during the transaction. On the SDK payment option screen, he would need to select the payment option and enter the respective card details. After this, SDK will launch the desired bank webpage (on the native webview) for further authentication.
Seamless Integration(Using Custom UI) - In this mode, the merchant needs to collect the customer card details on their own UI and pass them to PayU SDK by calling appropriate APIs which then launches the selected bank webpage. The merchant need not be PCI-DSS certified in this case (assuming app UI is not at all transferring card details to the merchant server). For further information on PCI-DSS certification please contact your Account Manager at PayU.