Am i being ripped off?

This question is around my riding mower, hopefully it is still relevant here. I recently had my tractor serviced and shortly after it was serviced it would not start. So I called the company back out and his notes stated that the tractor had gas in the oil and he had to drain the oil and replace solenoid /gas filter/oil filter, oil. He then charged me approximately $300, parts 100/labor 200. The tractor worked for a couple of weeks and then would start again. I would crank the engine and it would just grind and spray gas all over the floor. I eventually managed to get it started and started cutting the grass and then after two or three lengths of the yard it then cut out and smoke was billowing out of it. So I called him back out and he said gas was in the oil again and drained the oil again, changed the spark plug, changed the oil filter and charged me $45 in parts and 125 in labor.

I phoned him up to complain and he said all my problems are being caused by the fact that I use 87 gas, he said that this was a major problem this year and that all mower companies are experiencing problems such as this. I have used 87 since I bought the tractor 5 years ago and never had a problem. I would not imagine that using 87 could somehow cause the oil and gas to mix together. Is there anything to what he is saying or is he just ripping me off?