How can I add a handling fee to the orders?

as a percentage? instead of a flat rated number?

Hi,

You can add it as a shipping fee. That's possible with any of the payment tools you may want to use.

Example, if you want to use Paypal field, just drag it to your form, create your products, click next until you reached the other options section. Click shipping if you want to add a shipping fee. Unfortunately, shipping does not support percentage rate, only fixed price.

However, you may find this old method helpful specifically for the Paypal Standard. This has percentage rate support. https://www.jotform.com/help/188-How-to-add-Shipping-to-your-Paypal-standard-account

If you have further questions, let us know. Thanks!