why should customers pay an extra amount for check

Paid an extra amount for checking my bag at the airport. .safair is not the same as other airlines , one must also pay to check in bag..their making money out of their fine prints,why bill clients twice..if am booking a flight my checking in my bag must be included on the price advertised, very misleading,what exactly is one paying for if bags not included?? I didnt know that my bag will exceed I paid 250 extra even though I booked on line..I should have paid 150 that was on line..I dont have a scale at home hence I didnt add my bag..Id like to request my money back , this is a money making scheme.very disappointed.