You would always want to collect the money before mailing the package. PayPal is easiest. However, if the person who wants this does not specify that their payment of postage is a "gift" to you, a small percentage of the payment will be deducted by PayPal as a fee for selling an item.
I believe this would not be allowed as a PIF (Pay It Forward) item on the QB if you need the postage covered. You could probably offer it in the sales area, however. If you are concerned about the PayPal fee, I think you could "sell" each box for the price of whatever the real postage cost is (check, as I do think the flat rate boxes have gone up) plus a little more to cover the PayPal fee, and specify that postage is included. But check the QB rules to make sure.