How can I create multiple donation choices along with an Other amount option?

Gift amount:
$25
$50
$100
$250
$400
$500
$1,000
Other amount: $

Your form is close, just add an option to your gift amount field of "Other":

Then, conditionally show a text box to capture the other amount when other is selected:

Then add up the radio button and text box field in a calculation field:

https://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

Here is how the form would work the that set up:

http://support.jotform.com/form/63486625613966

You can clone it to your own account if you would like:

https://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL