This is for the protection of both you and your traveler. When you accept a delivery offer from a traveler, your credit card on file is authorized for the total amount, but not charged. The total amount it will be authorized for is the item price + Grabr fee + delivery fee to the traveler.
Your payment is put into Grabr's secure escrow account and only released to the traveler after you confirm delivery. This incentivizes your traveler to purchase and deliver your product as soon as possible.