It means Backpack will hold the payment for the traveler till the delivery is complete. Basically, Backpack acts as an escrow and keeps the money in its corporate account. The shopper is given proper documentation (receipts and Facebook/email notifications) of receiving the payment. This discourages moronic behaviors on the shoppers' part such as requesting products and then not showing up to take them and more importantly, paying the traveler and thus keeps the travelers safe. Also it keeps the shoppers safe, since the traveler don't get paid until they make the final delivery.