I was a bit drunk yesterday. I was trying to purchase 20 Giga Hash (2x10Ghs) hash power. By mistake I selected 20 Tera Hash (2x10Ths) from the drop down and clicked the link and it was gone. My friends told me this is the biggest bitcoin support forum. So if I tell my problem here, they can return my transaction.
As long as you haven't sent to the incorrect/unknowns address, there's a chance that merchant will refund you the bitcoins. Unless you bought on one of the cloudmining ponzis, then they may not be keen to refund.
Just contact the support and/or write in the official thread of the site you purchased from