As the seller has made an error - the shirt is not as described - all costs, including postage, should be refunded.
Wait until you hear from the seller. When she agrees to refund in full you will have to return the shirt first, by trackable means such as recorded delivery. This is so that you can prove that she has received it.
The refund should be by PayPal.
If your seller does not agree you can file a dispute with PayPal for Significantly Not As Described. PayPal will instruct you to return the shirt (by trackable means).
PayPal should then refund you.
An alternative is to telephone your seller, if you feel confident enough to do so.
But stick to your guns, you should not be one cent out of pocket.
Keep all emails and so on. Good luck!