How would you handle this?

I have a used 2006 VW Beetle that I bought certified “pre-loved” from a reputable local VW dearlership. Since I bought it I have only taken it to a local garage (STS Tires.) for oil changes. That is up until today when I took it to my local Firestone. The reason that I chose to take it to Firestone instead of STS is because two months ago I purchased fancy schmancy lightbulbs or my headlights and took it to STS to have them install the headlights. Long story short, they never did the job correctly and I kept having to take it back to them. Hoping each time that they had hooked the lights up right this time. Finally I got frustrated and took it to Firestone and they fixed the lights for me no charge. Today, I needed an oil change and I took it to Firestone. My new favorite mechanic at Firestone pulled me aside and showed me the oil pan. The oil plug was stripped from being turned to tightly. I had him hold off on changing the oil for now. I called STS and told them about my situation and the Manager goes “yeah” that happens the oil pugs get stripped. I am fuming! I explained to them that I bought the car from VW an that they were the first and only people to change the oil since I bought it. If the plug had been stripped when I first brought it to them after purchasing it then why was I not told about it? ( I know…Because it wasn’t stripped!)He told me to bring it in tomorrow. My worry is that at this point how do I know that it will be fixed correctly? Clearly they have had know idea what they are doing in the first place! Not to mention that they are trying to act like they didn’t do it. I am fuming! Help! What would you guys do???