Put the $$$ on top of the box, then your cadet will have the cash right there to pay for the pick up.
Of course, if you ship thru USPS, that's not a problem.
Those flat rate boxes are EXCELLENT! They hold up to 70 pounds (large) and go ahead, STUFF IT! We sent our kids tuna packets, beef jerky, and high protein snacks and things they couldn't get for themselves at the C store. Then, fill in the gaps with candy, gum, money, socks, gloves, hand/foot warmers (greatly appreciated later in the season), toothpaste (travel size), etc. Stamps, envelopes, stamped postcards, put it all in there!
Only caveat, we were not a "once a month" package shipper family. We budgeted $100 per box filler then added the shipping cost. Multiplied by two, well, you get the idea.