This is not Paypal it's a Self Built Payment Page, wherein customers can checkout there purchases from an e-commerce site. What I want to do which I have no idea how is to Send a URL Link to the Customer who has not paid. I don't know how will I generate that URL, When the customer clicks the URL he/she will be redirected to the Payment Page and choose the Payment options. Any help/advice will do. Thanks.
You could generate a unique token to every payment in the database, then you just include this token in the URL in a way you can compare and search for that payment in the database.
Something like http://yourwebsite.com/payment?token=a8sd56as5d6as5d6as5d6sd for example.
If this doesn't answer your question, please give more information.