Paypal - Can we cancel a payment made n get our money back?

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I made a payment three days back from Paypal. I bought some ad links for that. I got confiormation that the payment has been made. But the problem is that site has not placed my ad till now - I mailed them twice but they have not replied and clarifed the issue still. So I want to know, what can i do abt it? Can i call back the payment that i made? If yes how, if no, what else can be done. Pls suggest, thanks.