Most sellers charge 'shipping and handling' costs. There is more to that
than just the cost of 'actual postage'.
Boxes cost money. Tape costs money. Packing materials cost money.
If you use PayPal, they take a percentage of the entire sale - including
shipping charges. So, if it costs you $3 to ship, and you collect $3
from a buyer, you're in the hole 9 cents or so already.
As long as a seller is up front about what the S/H fees are on the front
end, I have no gripes. If something is worth $100 to me, and shipping is
$30, my maximum bid is $70.
There are a lot of buyers who think you should only charge what the
postage value on the label is, and if you print $2.54 in postage and
charge them $3 to ship, you're ripping them off!
Never mind the mailer was a quarter, the label was a dime, PayPal took 9
cents... and the packing materials were another quarter.