When the submit button is clicked it goes to the wrong page

When a user selects a purchase, everything works as it should, i.e. they are directed to the PayPal site for payment. If they click on the submit button (titled "Go to secure payment page") without clicking on an item to purchase they are taken to a "Thank You" page. If they click the button without having selected an item, I would like them to be directed to a page that tells them, "You didn't select an item to purchase"

I figured out a way to make it do what I want.

Just make the payment field required then user won't be able to submit without selecting an item in your purchase menu.