Those signs mean squat. The repair place must take reasonable means to protect your property. If they knew that they had a rat problem and didn't abate it, a court would side with you. The problem is reality steps in.... they'll point to their sign and most people will think it means something. Many places will dig in and not do anything for you, then you have to decide if you want to take it to small claims court.
Best way is to explain the situation in a calm and intelligent manner, and you'll find out if the repair place does the right thing or not. Explain how you have dropped over $4k at their place, and will continue to use them in the future. If its apparent that they are not going to give sufficient relief, you can always escalate the situation. Is there a local television station that has a crew that works on investigating stuff like this? In Toledo, we have 2 TV stations who love stuff like this. I used one of them several years ago when Menards tried to keep $300 of mine (long story). When TV-11 called their corporate HQ, the local store was very happy to give me my $300 cash back. Just a thought....
A $100 back on a $500 bill is a good start, offer to be happy if they meet you in the middle and pay half.