2003 Ford Windstar transmission at 77,000 miles

I just purchased a 2003 ford windstar in Feb. And yesterday, I tried to pull away from the gas pump at the service station when the car was in drive, the car wanted to go in reverse, so I tried all the gears for 10 min and it still wanted to go in reverse. It finally went into drive after the 10 min, and the transmission fluid was low, but it was not registering inside on the panel as being low and there was no evident leaking that I could see outside. The dealer wants to charge me 250.00 for a clogged oil filter, but I live on a fixed income and I am afraid they are trying to take advantage of me because I'm a senior and a female. Can you tell me if this is the problem or are they trying to rip me off?