If a contract states it must be cancelled in person or by certified letter can other means of correspondance be used in lieu of the certified letter?

I joined planet fitness over 2 years ago in Baltimore. After my contractual term was up I went into the gym and cancelled my membership. Without me noticing, due to the small charges, they continued to bill me for a year. I noticed when I moved, closed my associated account, and started receiving bills in the mail. I did not receive a receipt for my cancellation. I contacted them immediately and was told my membership was cancelled but I would not be refunded the money they had been billed, even though my gym attendance records support my claim. I filled a BBB complaint in response and on top of this I stated that I was afraid they would continue to bill me even though they told me my membership was cancelled as I did not receive a receipt. They assured me over the phone they would not. They have now been billing me monthly and told me over the phone they would not cancel unless I paid what I owed. I believe my BBB complaint provides sufficient notification of my cancellation.