Not sure about checking out by itself, but Chrome has autofill options built in.

Ya, I would like a program where I enter in all the info, and based on that info, it fills out all the shipping and payment info, and checkout itself.

Chrome, you have to manually click on the dropbox. I would like it to be automatically filled out when the program encounters such forms to be filled out.

It would probably be easy enough to write an extension that lets you pre-populate the form fields that are likely to be present in most check-out systems.

That wouldn't automatically submit the form and complete the checkout for you, but I don't know why you would want that in the first place.

Do you mean like a desktop program? Maybe but a web browser I doubt it, imagine if you could do all those steps without authorization from the user. Then you would be in big trouble from those that don't understand the system.

Ya, a desktop program.