Authorize.net integration: Add a service charge

I need to build a payment form for a client integrating with Authorize.net. How can I automatically add a service fee to a form so that if, for instance, they enter $500 as the payment amount that an automatic $15 gets added and therefore $515 gets charged?

Thanks,

Ron

You can add a shipping charge of $15 and label it as "Service Charge".

For example:

Demo here:

https://form.jotform.com/80115759236963

Thank you, but the problem is that we are not selling products, we only
sell services. Is there a way to include an automatic $15 convenience fee
for a shopping cart that only sells services?
...

If you want to setup a recurring service for sale, you should select "sell subscriptions" And for the first charge ask for 515. Thereafter they will be charged 500 per subscription period.

For example:

Hope that helps!