Check engine light coming back on within 24hrs of repair

I finally took my 2010 chevy cobalt in to fix an issue with the EVAPS system since the state of Colorado requires your car can't have those codes pop up if you want to renew your tags. I took it in and two days later they finally found the issue; an animal of some sort crawled on top of my gas tank and chewed some wires. I was charged almost $300 for labor, soldering the wires and a smoke test. Not even 24hrs later the check engine light comes back on with the same codes popping up. I called them back and they said that I could bring it back in for them to further inspect it but they will continue to charge me for labor. This is one of my first experiences at a mechanic and want to make sure I'm not being taken advantage of. I feel like I shouldn't have to pay for anything besides the diagnostic because they didn't fix anything. Am I within my rights to dispute this? I understand they worked on stuff and it took time, but I'm still left with the same problem I brought to them.