Welcome to the Ranch
I think part of the problem is that you have tried to write a GUI rather than an application.
You should write the application and then add the GUI as its display. Now you have over 200 lines of code most of which don't have anything to do with the problem.
Do you mean you want to add the current purchase to a previous purchase to make a total? In which case the only total I can see appears to be a local variable in one of the action performed methods. That will never add anything.
Program4 is a really bad name for a class. The variables should not start with CapitalLetters. Use camelCaseNames. Most of those Components are fields in the frame class; change them all to be local variables in the constructor/setUpGUI method. Don' use null layout. Never use the
== operator on reference types. Least of all Strings, because it will sometimes return true.
There must be a better way to set up your combo box than that. Yes, create an Animal class with type and price fields.