In some cases (extra) checked baggage can be added afterwards.

1. Add baggage on the website of the airline

Please go to 'Manage my booking' and fill out your details (you booking reference can be found in ticket e-mail). If available, the option to add baggage can be found here. (Extra) baggage can be paid for directly. 

NOTE: for some airlines it is only possible to add luggage during check in at the airport (United Airlines, Delta Airlines). The airline guarantees at least one piece per traveler.

2. Add baggage at the airport
At the airport go to the desk of the airline and ask the steward to add baggage and pay for the baggage directly.