It may not be monthly but there are companies that will monitor your levels and schedule refills as needed. If you're getting refilled per month you probably need a bigger tank. Most people are using propane for heating and don't need it refilled in the summer.

There are also options to spread the cost over the year so you're making payments each month. That is sort of a subscription but when I think subscription I think of a service you're charged for whether you use it or not and not financing.