How to make multi payment fields?

I have an existing form using PayPal where people are required to select one of three options. I'd now like to add a non-required (optional) field where, once they've clicked on one of the 3 required fields, they can also elect to add this option (which should also be linked to PayPal) if they so choose.

How do this? As I understand it form will only allow me to have one payment field.

Yes you are correct that form builder only allows single payment field but there's a work around for this. What you need to do is create a new form that has a paypal payment then use a conditional rule to hide and show fields.

I have cloned your form and made a sample. Please check this out http://form.jotformpro.com/form/42465179710962.

For more info about how to make a basic multipayment form please see our guide in this link https://www.jotform.com/help/182-How-to-Create-a-Basic-Multiple-Payment-form.