EBAY 101
Why do I charge shipping? SO YOU CAN SAVE MONEY
BUYING MULTIPLE cards saves you money.
If I list an item with free shipping, I have to take into account the fees and postage.
A $1.99 free shipping listing nets me $.76 for the item.
If you buy 2 items, you pay $3.98.
eBay has a minimum list price >> $.99
I charge $1.25 shipping to cover postage and fees. Additional cards cost $.15.
So you buy 2 cards and pay $3.38 total. You save $.60 ($.30 per card)


YOUR COST
Free S/H = $1.99 per card
VS
2X Card + s/h = $1.69 per card.
10 CARDS ($9.90 + $2.60 S/H = $12.50 ) = $1.25 PER CARD
WE BOTH WIN.

SOME CARDS JUST AREN'T WORTH $.99 LET ALONE SHIPPING COSTS.

SO I HAVE LISTINGS WITH THE $.99 MINIMUM BIN.
I run a sale of say 50%.  
Card price is $.49. s/h is $1.25 You pay $1.74.
you buy 2 cards.
pay (.49X2=.98)+(1.40 s/h)= $2.38
YOU PAY $1.19 per card
YOU BUY 10 CARDS: pay $4.90 + $2.60 s/h = $7.50  ($.75 per card)
THE MORE YOU BUY, THE MORE YOU SAVE

I know sales tax is also added to the equation, but every state is different.
I left it out to make the math easier to understand.

eBay fees

item + shipping (if any) + tax = sum
sum X 13.25% = fee
fee + $.30 (transaction fee) = eBay cost
(sum - cost) - tax = seller sub net
seller sub net - shipping  = seller net

$1.99 item / free sh / 6% tax
$1.99 X .06 = .12 = $2.11
$2.11 X .1325 = $.28 = $1.83
$1.83 - $.30 = $1.53
$1.53 - .12 = $1.41 - $.65 sh = $.76 seller net