When was the last time you added money to your account? If you are unsure of the exact date or simply do not remember, you can log in to your online account and see your account history. https://www.paygonline.com/websc/index.jsp
Although it may not necessarily be the "cheapest" way, you could consider adding $25 to your account every three months or so to keep it alive. Remember that $25 adds 90 days to your account expiration date, which is the equalivalent of three months. Even after the three month time period, your account still has a 60 day "grace period" until it is officially closed and deactivated. Altogether, this will allow your account to be safe for 5 additional months.
Another more expensive alternative would be to add $100 to your account so that it adds a full calendar year to your account expiration date, but $100 is a lot of money and you may question if it is really worth it.