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I learned to read errors with someone pointing them out to me, and I find I can read errors myself fine now.
I'm sure they'll figure it out down the line.

You seem to have written "JTextField paymentField = new JTextField(10);" when you already have "private JTextField paymentField;" at the top as well.

Alright.

Well I copyed & pasted your code, and I found the problem. You didn't create "paymentField = new JTextField();", in my IDE, your paymentField comes up as inactive.

Try this:
Instead of putting variables such as "double payment" inside your methods, put them with your instance variables, like:

"private double payment;"

set them to 0 in your constructor:
...

[removed - sorry, accidental double-post]