First time I have ever had this happen (30 years in business). Back charge for corrective work is conveniently $100 less than the balance on the contract. I asked for specifics on time spent, work that needed to be done on what specific areas - no response. I agree some work had to be done, and expected about 54% of the amount they ended up sending me notice of. For months I have asked for information on work done and time it took (not looking for daily logs but at least something).