Budget airlines aren't a scam — the base fare is genuinely low — but the business model depends on a meaningful share of passengers paying for add-ons that used to be included. Knowing exactly which ones matter avoids most of the sting.
The carry-on size trap
The single most common unplanned cost on budget carriers is a gate agent measuring your "carry-on" and charging a bag fee on the spot, often more expensive than pre-booking a checked bag online would have been. Always check the airline's exact carry-on dimensions (not a generic "carry-on size") before you fly, and measure your bag at home.
Seat selection is optional, priority boarding usually is too
Unless you specifically need to sit together with someone, skip paid seat selection — you'll be assigned a seat for free either way. Priority boarding matters only if overhead bin space is genuinely limited and your bag needs to fit; otherwise it's a low-value add-on.
Airport transfer distance is a real hidden cost
Many budget carriers fly into secondary airports well outside the city center. Factor in the ground transport cost and time before comparing the fare to a full-service airline's city-center-adjacent option — sometimes the "cheap" flight isn't cheaper once that's included.
Takeaway: Measure your bag against the specific airline's carry-on limits before departure day — this single step avoids the most common and most expensive budget-airline surprise.