Check your bags at the appropriate airline ticket counter when you arrive and then proceed to security.
I have heavy box i want to drop at ticket counter before returning rental car. Can I?
Most of the airlines have a number of boarding pass kiosks. You enter your flight confirmation number, or your last name, or the credit card you bought your ticket with. The system will then work with you to get seats (or confirm existing seats), upgrade, check luggage (to include strollers, skis, etc) and will then print your boarding pass. Most airlines have personnel to help in this process if needed!
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