Buried Propane Tank And New Fees Due To Low Usage

I have a buried propane tank in my yard that has been there since before I bought the house. It is owned by a propane company nearby. I never signed any contract or agreement with the company but when we moved in and first had them fill up the tank, it was explained to us that they charged us a higher rate (per gallon of propane) than market because they owned the tank, not us. Also that they were the only ones able to fill it (they have a lock on the tank).

We were paying for so much propane that after the first year of living in the house we had a heat pump installed. The heat pump has been amazing and now our furnace barely runs--this means we now use very little propane (for our water heater only) and haven't needed a fill up in over a year and a half now.

Last week the propane company emailed us and said that they're now going to begin charging a monthly rental fee for the tank since we haven't used enough propane. They said they'd sell us the tank for an extremely high price (which really makes me think they don't want to sell it). They said that if we didn't like the new arrangement, we would have to pay to have the tank excavated they'd come pick it up. Also that if we didn't pay the fee, they'd put a lien on our house. This really sucks, because now a lot of the money we've saved with the heat pump would still go to the propane company--even though we barely use it.

Can they legally just change terms on us like this? Also, would we have to pay to have it removed--They are the ones who changed the pay arrangement on us? Could they actually put a lien on our house for this? Any tips for how to respond? If I could put the excavation costs on them to remove, then I wonder if they'd come down to a reasonable price and I could just buy the tank from them.

Location is Massachusetts- USA