how to donate additional funds on top of payment

Can we add options to Donate additional funds or round up to nearest dollar?

Yes, this is also possible.

If you want to give your user's the option to add donations to an already existing payment you can insert a Form Calculation widget into your form and use the user's donation input to calculate a total. However, this will require that you set up your payments to be 'Defined by user' and then use the calculation widget to feed the total into your payment integration.

You will also need to create your products individually as Single or Multiple Choice fields and then assign calculation values to them so that you can calculate a total with the calculation widget.

This process does take some additional steps but it is possible. I went ahead and created a sample form to mimic this functionality. You can visit this form via this link: https://form.jotform.com/201196075642050

You can clone the form for yourself if you would like to take a closer look at how I was able to set this up.

Related User Guides:

How-to-Perform-Form-Calculation-Using-a-Widget

How-to-Assign-Calculation-Value

How-to-Clone-an-Existing-Form-from-a-URL

I was able to clone and take a closer look at the form you created, but I
could not find the widget you used to for the Donation field. When I
opened that box in the builder it says "Number Properties" at the top of
the gear box, but I can't find the name of the widget that allowed that.
Help!
...

The donations element is actually just a Number field. That is where you will request the donation amount from your respondents. There is no special donations widget that you need to use.

This number field will be used inside the Form Calculation widget to calculate a total. You can actually hide the calculations widget so that your respondents only see the total in one place and not the other. I left the calculations widget visible to help you see how the calculations happen in real-time.

Here is a quick screen recording to show how you can make some changes to the form and to show its functionality.