Complaint Review: airgas

I had them fill my 5 lb. gas cyliner with co2 gas 3 months ago and they charged me 13.00 a little high but acceptable, then on 11/11/13 they charged me 26.95.  I know they did on purpose because they had me sign a contract saying I will pay the bill or not get my cylinder back.  I know I'm small customer but this is a blatant case of gouring a customer because they can, Shame on you Airgas!!!!!!!!!  P.s.  I call up another company and they said they would fill it for 10.00.