"buyer to pay postage"

The buyer did  pay postage, the postage shown on your listing which would have been  $13.97 to my state.

No doubt you miscalculated and  it cost you a lot more so you had to pay the extra yourself.

Is that what you mean   in relation to  the $68 ?

"It sold for $121 I assumed I would receive $100 less eBay commission" 

But even if you'd  put a postage cost of  $50, the final value fee would have been  based on the total  amount of the sale, including postage.