Well, I have to wonder about just what's included in "annual maintenance" that you can't do yourself. Aside from the air filter, which you certainly can buy and replace yourself at lower cost, what is there? Check that the system comes on and cools the air? Check the refrigerant pressure as a check for a leak, that you'd know about if the system doesn't deliver on a hot day? Perhaps someone here can provide a more positive explanation of the need for having an annual maintenance service call.
My own heat pump system is of course reversible, so I get AC in warm weather. I buy/replace my own filters, and I monitor the system myself, which runs in one mode or the other most of the year. "Monitoring" mostly consists of observing whether I get heat in the winter, AC in the summer. If something seemed wrong, then I'd explore in more depth, perhaps calling in an HVAC company to do things I can't. But after four years of service-free operation, having an annual expense to tell me "Yep, she's a workin" doesn't seem appropriate.
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