Yes, a backpack can be considered either a Frontier carry-on or a personal item, but it depends on its size. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) A backpack that fits the smaller dimensions of a personal item (14"H x 18"W x 8"D) is included for free, while a larger backpack that fits the carry-on dimensions (24"H x 16"W x 10"D) requires payment and must be purchased at booking for the lowest price. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) To avoid fees, your backpack must be small enough to fit under the seat in front of you, and Frontier is known for strictly enforcing these size limits.