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 there. (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). 

2. Add baggage at the airport

At the airport go to the desk of the airline and ask the ground staff to add baggage and pay for the baggage directly.