No returns does not mean no refunds.
They are even spelled differently.
Tell the buyer to return the dress for a full refund.
If she demands a Return Shipping Label, and she is in your country, send her a RSL.
You will control the service and tracking used, and therefore the cost of shipping.
When you get the dress back, return her original payment.
This is business, not personal.
If she goes to eBay with a Dispute she can choose Not As Described or Not Wanted/Buyer Remorse.
Most buyers go with NAD.
EBay will require you to send the RSL.
If you don't, she keeps the dress, eBay refunds her, and then eBay charges you for the refund.
Oh, and you get a Defect.
And the buyer can leave feedback.
Tell her to return for refund.