Just Energy Switched my energy rate to variable with NO NOTICE, doubled fees for six months. Pittsburgh Pennsylvania Nationwide

I have a rental property for which my tenant pays me the electric bill which is in my name. I have noticed over the past few months that the energy cost was getting higher and i thought it was due to the cold winter and higher enegry usage. I called Duquese Light last month and they said call your energy supplier which is JUST ENERGY. In December they had changed my fixed electrical usage rate to a nearly DOUBLE variable rate with NO NOTICE (total extra fees amounting to about $1500.00). I called Just Energy and tried to get reimbursed, they reviewed my account and said they sent me a POST CARD in the mail when the rate change occured (which i have never recieved). I have gotten no reimburement and they offered to send me a $20.00 visa gift card which i declined. If anyone can offer any information about anything i can do to try and reclaim some money that wouuld be great!!!!