Although the seller IS responsible for getting you the item or refunding your money, they can not be held accountable for any goofups the USPS make (and we know they sometimes goof up).
If she can prove she shipped it in a timely manner, and it just was delayed by the PO in getting to you, please do NOT take it out on the seller by leaving neg FB or dinging their shipping stars. I hate it when that happens to me as a seller, all I can do is ship it quickly and hope for the best, then it is in the PO's hands.