We ask you to pay for the product because that works best for both the shopper and the traveler. There's basically 2 primary risks involved, the traveler snatching the item for themselves and the shopper not paying for the item. The way we solve it is by taking the money from the shopper as soon as the deal is confirmed and paying the traveler when the delivery is made.
You should not be concerned since we will notify you to buy the product only after we receive the payment from the shopper. After the shopper picks up the product, he/she will show you an email/sms which will contain a code and as soon as you enter it in the website, you will receive the payment in your preferred way.
But if you are still concerned, you should apply for the trusted traveler program where you won't even have to purchase the product. Backpack will purchase them for you and send them to your address. You'll just have to carry and deliver!