The reason they send the bill out late (they sent mine out on the day it was due) is so if you cancel they can still bang you for a week or two for coverage you've cancelled.
Give them the date you wanted to cancel and explain the bill was late. They will honor your requested cancel date and refund any money owed. I speak from experience.
Prepay for 5 months at a time. They have a 5 months for $25 promo. Then put it on your calendar to renew and get the same deal in 4 months and 3 weeks.
If they don't catch on. A friend tried this and they wouldn't honor the deal because he wasn't a first time subscriber. Gaming the system rarely works. I can't be bothered to waste so much time to save a few bucks.