I've been charged for checking my luggage.

The requirements of the carrier regarding hand baggage must be included in the contract of carriage.

If your luggage does not comply with these requirements, you will be subject to the provisions of the contract of carriage. It is the passenger's responsibility to ensure that his/her luggage complies with the company's requirements. 

Regarding checked baggage, airlines may establish an optional supplement on the price of the ticket. However, in any case, the final price of all services offered must be shown or indicated before the final purchase is made.