First off, I let the guest contact me, unless I have discovered it within 30 minutes of their leaving, and think they may still be in the area.
Then, if contacted, I advise the guest that I can fit their item in one of the three "flat_rate_shipping" boxes. They're supplied by the post office at no charge, and regardless of the weight, it is one price. I know before I go to the po how much the shipping will be, and I charge their card first. So if a charger was left behind, I know it will fit in the smallest box, and it will cost the guest $5.60. If they left some clothing, it may have to go into a large box and the cost is $17.45. I don't have to come up with a box or tape. Label it and drop it off - that's it. For this reason, I don't charge a "handling fee" or charge extra. The PO is on my way to most anything I do daily, so it's a quick trip for me..