I am new to paypal integration,
Requirement (paypal USA): Admin should be able to generate paypal invoice from website admin panel. As this is service invoice and charges will be different depending on client requirement. And User, from website client area can receive paypal invoice and pay it online.
I ve read few documents of paypal api but couldn't decide which flow/API to use.
Please suggest best way with least coding to integrate paypal with my website.
You can use the Invoicing API for that. My PHP class library for PayPal will make this very simple for you. It comes with fully functional samples and ready-made templates so you can get calls working very quickly and easily.
For example, take a look at the CreateAndSendInvoice sample. All you would need to do is replace the static values with your own static or dynamic values and it would create the invoice in the PayPal system as well as send the user an email to pay the invoice.
It also comes with all the other calls like CreateInvoice, SendInvoice, DeleteInvoice, CancelInvoice, etc, so you can easily make any call you need for a complete PayPal invoicing system.