You should add baggage to your flight at the time of booking or at least 24 hours before your departure via the airline's website or app. In 2026, the pricing for checked luggage is tiered: it is almost always cheapest to pay during the initial ticket purchase. If you wait until the online check-in window (24 hours prior), the price often increases by 10-20%. The most expensive place to add a bag is at the airport check-in counter, where airlines like Spirit, Ryanair, or even major carriers like United and Lufthansa may charge nearly double the online rate. Adding bags early also helps you avoid "gate-check" fees, which occur if your carry-on is deemed too large and must be moved to the hold at the last minute. Furthermore, pre-paying for your bags online saves significant time at the airport, as it allows you to use the automated "Self-Service Bag Drop" kiosks, often letting you bypass the long lines at the full-service ticket counters entirely.