I hope this one sticks around.
I could see charging a buck or two more, and then a buck or two more to get a little extra. I really don't see charging 6.95 for a 4.30 priority ship excessive. That would cover the box I need to buy, the time it takes me to go to the PO (because I don't trust the PM to pick it up) and my time. Fair, I think.
Now, I just sold an item to a CAG here for $21. I wanted to get $15 for the actual item. $15+4.30+.74+1.05 box = $21.09. As it happened, I already had a box, but I always charge a buck for a box, or $.50 for an envelope. If I were on eBay, I would charge $6.95, or if I was unsure about the weight, I would charge $9 flat rate.