International Selling/Shipping

[penny1024]

Hello, I usually on sell and ship domestically (U.S.) but someone from the UK really wanted to buy something from me, a pair of designer shoes. I insured the item and shipped them to her, however now customs wants her to pay tax and it's a lot of money, apparently almost $300. How could I have avoided this? Is it my fault? I did insure the item before I sent it because it was pricey. I told her she could just have it returned to sender and I'll refund her money. Was it my mistake or did she not take into account the customs fees?