How to collect custom amount invoiced payments through PayPal

I would like customers to be able to pay their invoices online.  They receive their invoice by printed mail.

I need to add a surcharge to the invoice if they pay online.

So i need them to enter the amount of the invoice, and then I need to automatically add a service fee, to come to a total that they agree to pay via paypal.

How would I do this?

Thank you

You can do this by (1) having a Number field on your form where users would enter their amount manually, (2) having a Form Calculation where you would calculate a total amount based on what they entered on the Number field, and then (3) pass this calculated amount to PayPal for payment.

Step 1: We add a Number field.

Step 2: We create a form calculation, and set it to calculate the number field plus our service fee.

https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget

Step 3: We pass the calculated total to the payment field:

https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field

Please see the following demo form of this kind of setup:

https://form.jotformpro.com/form/70064379982971

Please try it out, and let us know should you need further assistance.