Several comments:
For first class mail, the price is the same whether it's going to your next door neighbor or if it's going to Alaska. Up to 16 oz/15.9, the item can go first class.
At 15.3 oz., the seller should have paid for a 16 oz. label. (It would show as 15.9 oz; for some reason, a 1-lb. first class parcel doesn't show 1 lb.) The fact that she only paid for 8 oz. and didn't get caught merely indicates that she cheated the post office and they didn't catch it. They (USPS) handle thousands of pieces of mail per day and can't weigh every one of them.
Had you been charged for the postage due, you could have forwarded a picture of the proof and if honest (which clearly she isn't!), she'd reimburse you.
ETA: I just reread your post. The packing materials brought the weight over the first class threshhold and over 1 lb. She should have paid for a 2-lb package! Ugh!
I'm curious. What did the listing show as the shipping method and how much did you pay for shipping?
Listing had Standard Shipping and I paid $6.00!
And here I was assuming USPS weighed all packages one way or another...oops!