Generate Invoice number & show amount when using PayPal

I am integrating PayPal in my website But I have some issues in it.

O开发者_运维技巧n this page, if you click on the PayPal button here you will see one box.

I need two boxes to put amount and invoice number. How can I do this?

I hope Paypal will take you these information [Amount(if not set) and Quantity] in this step, the description, name etc depends on you.

The invoice number will get generated and can be seen once the transaction is completed in your account