How much is the postage? I'd probably eat the cost if it's $5 or less. And split the difference if it is more. The problem is customers EXPECT people to go above and beyond. Easier for corporations
Also, that's kind of jerky of her to ask you to resend it for free! I would think if she told her friends that she was mad you didn't cover shipping when she mucked up, her friends would agree with u
It's a tough one. Seeing s/he is at fault I would offer to send it again if they paid and if they weren't happy with that offer to refund the purchase. If it is overseas and costing $18+ then you cannot afford