There is a couple ways you can proceed. Since the fault rests on your buyer for not providing a correct mailing address, I would suggest to her that she can repay the shipping and you will resend or that once the item returns back you can reimburse her for the cost of the pets. I would in no way refund her the original shipping costs.
As far as the item being returned to you it may take a bit longer as USPS uses the slowest method possible when items get returned to the original sender. So I would sit tight for right now and message her back explaining that to her. If she does open a case, don't worry. If you shipped to the address that the buyer provided with the payment (you'll see this on the transaction's Order Details / the PayPal Transaction Details page), call eBay, provide the tracking number and mention that the buyer provided an invalid address. They may escalate and close the case in your favor.