Optional donation area on registration form

I am working on a registration form for a conference and would like to add an optional donation field that will be added to the cost of registration. I have options the user should choose from for their registration cost, and I want them to be able to add a custom donation to their total cost. How would I do this? 

You have Paypal in the form. I presumed this is where you'd like to add this option. If yes, you have to add a product with one 1 as price. You can name this "Donation".

Next, set up a Quantity property and select Textbox type so that people can enter an amount. That amount will be multiplied by 1, so to have a fixed donation value.

The quantity box is too small and the price will be visible. The width can be adjusted with Custom CSS codes. The price can also be hidden by CSS. We can help with this.

If you want, we can add this to your form. We only need your permission to edit your settings.

I've added it to the Paypal area on my form, but I don't know how to fix that quantity box, so you have permission to fix it.

I added the needed CSS codes on your form to customize the Donation item/product.

I suggest you test the form so you can evaluate.

Will this then add the optional donation to the total that is due?

Yes it should.

We will resolve the other question here https://www.jotform.com/answers/1748817-Add-a-donation-option-to-pay-by-check