How to allow the user to fill the amount that he/she will pay?

I sell service and not products. I need to set up a payment form that asks for an invoice # and lets the client fill in the dollar amount. Is there a mechanism to allow the client to fill in the amount they are sending?

Hello,

You can use a PayPal payment field in your form and you can change the payment type as "User Defined Amount" as you can see the screenshot below:

So that, the user will be able to pay for the amount that he/she has entered.

If you need further clarification, let us know.

Thank you.