It's not clear when you stopped the transaction.
If you confirmed the purchase but not the payment, the seller can open an Unpaid Item Dispute and put a Strike against your selling account, which could make it more difficult for you to buy in future.
And I suspect that is the case, because otherwise the seller would not know your ID.
Answer her by explaining the problem with the shipping charge.
Does the seller use the Global Shipping Program?
Does she know she is enrolled in the GSP?
Let her know how much the import fees under the GSP are. Most sellers have no idea and many don't know* they are offering the GSP.
Ask her to send you a Request for a Mutual Cancellation giving the reason "Buyer changed his mind."
If she does this, agree immediately. Leave positive feedback or no feedback. (Remember she could have hit you with that UID and Strike.) Both of you are now free of the transaction and the seller gets her fees back, which can be upwards of 20% of your payment.
If she was unaware of the cost of the GSP and wants to go ahead with the sale, there is a way to make both of your happy. Come back and ask.
*If they do know, they use it because shipping outside the USA terrifies them. But many US sellers were signed up without their knowledge.
