I know how because i have gotton one with another asset.
Well you need to be in direct contact with the developer preferably by mail,
and Caitlyn who is the Unity3d store manager.
Caitlyn Meeks-Ferragallo <caitlyn@unity3d.com>
If you cannot get in personal contact with the developer you still need to speak with Caitlyn she handles
all matters in the store..
IF you are able to get in contact with the developer, and you agree on a refund,
the developer needs to mail to support@unity3d.com
a mail in this format
Hello,
please refund the customer <COSTUMERNAME> which had purchased the <ASSETNAME> plugin on <PURCHASEDATE>
His invoice number is: <INVOICE NUMBER>
Thank you.
Developer signature
so you need to find these things for the developer first, then contact him again.
It's all on the unity forum, and in terms of using the store.. (READ THEM)