Product received but buyer has opened a case

[RedWineAllMine]

Ok, so I know how Ebay works, but I have never came across this before. Advice much appreciated!
So, I sent a product within another box, (the box wasn't for the product, but I put it in the box to protect the product - as you do)
Buyer has now messaged saying I sent her the wrong product and she wants a full refund. However the product is in the box if she just opens it she would see! She hasn't opened the box. I messaged her back and told her this, and told her she needs to close the case also, as she has the product. She hasn't yet closed the case.
So what shall I do? Ring Ebay? Let the case run until the last day then get Ebay to step in? The box was for a entirely different product, and she included a photo of the box on the case details.
Thanks

[Inforthelonghaul]

Sounds like she’s trying to do a sneaky one and keep the ‘wrong’ item and get a full refund too. You should definitely call EBay, have you also explained this on the case messages so that eBay can see you’ve told her?

[RedWineAllMine]

Hi thanks. Yes I told her the product is in the box, I was very polite about it all, which her message wasn't.
I have sold the same products, within a box, so I can prove the weight will be around the same etc if that made any difference.
I understand I can accept the refund, and I only get charged for the postage slip if she uses it etc, and she would need to post it back. If she doesn't post it back the case would close in my favour.
She only messaged me the other day, so it is still early for her to reply etc. But I'm just panicking as I have 100% positive feedback.
But I'm just wondering what the correct steps are to take form here.