Having had the pleasure of mailing you [name of magazine] every month for the past three years， we were wondering if perhaps you haven't realized that your subscription ran out in [month]
We mailed you our [month] issue and have not yet removed your name from our subscriber's list inasmuch as we felt that you may be unaware that it is time for your renewal.
We have enclosed our renewal subscription card for you which provides for 12 issues of [name of magazine] at the low price of $ per issue， or $ less than the newsstand price. As you can see， this represents a substantial savings.
We are looking forward to hearing from you soon.