Q: Accidental Purchase music on itunes

All sales are final

There's no "procedure" and in principle all sales are final, but it never hurts to ask. 

My advice would be to immediately email the iTunes store and ask very nicely:

http://www.apple.com/lae/support/itunes/contact.html

The worst they can do is say no - which they probably will, but hey, who knows.

YES YOU CAN !     The person who answered is ... WRONG

#1 Log on to your account to review purchases

#2 Select the purchase by click on the arrow on the left side of the line item. 

#3 A detail of the purchase will appear.   

#4 Click REPORT A PROBLEM next to the line item.

#5 From the drop down menu,  select ACCIDENTAL PURCHASE

#6  Click DONE