Looks to me like she got the other pair cheaper and figures the baby can grow into the size 2. Just the whole tone of her email makes me agree with Lisa that she's no newbie. She really wanted those boots and probably didn't think she'd win both auctions so she's unfairly complaining and hinting about the neg she's going to give you to scare you into taking them back AND paying return shipping.
I would tell her a deal's a deal, you KNOW they were brand new and that she bought an identical pair at the same time. If she thinks she needs to leave you a negative because you won't pay return shipping, then so be it. One negative lost in the sea of all your positives won't look nearly as bad as her one negative out of 11. lol Blackmail her back. Stand your ground and be firm. Make sure she knows that you have investigated and seen that she won an identical pair for less money and ask her if that's the real reason for her complaint.
Just my two cents. People like that irk me.