How do I have a conditional amount to be paid?

i.e. the amount owed is dependent on the amount ordered

If the standard payment tool is not flexible enough, you may consider using standard fields and custom calculations based on them.

1) How to perform calculation in the form.
2) How to assign calculation value.
3) How to pass a calculation to a payment field.

Take a look at 'Calculated amount based on user selections and inputs' section of the next article to see it in action.

Mastering Payment Form Integrations with JotForm

If you need any further assistance on this, please let us know.