Commisions on cashout from earning sites

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I've recently noticed some extrange things in that. We all know, every time we cashout from a PTC site, or for instance any other online earning site, they discount us a small fee for transaction cost. But I've cashed out from NeoBux to AlertPay, and they charged nothing!! A few days latter, I cashed out to AlertPay from PalmBux, and they cost was 9,5%, 3,5% if it were to PayPal. I can't remember the OnBux rates, but they were similar. So this makes me think Who is really charging that? NeoBux or PayPal? And why are those rates so different from site to site, from 0 to 10%? Maybe the site is trying to scam us a little money (all cashouts sums to a great amount) or it's the PayPal bussiness. And in the latter case, would not be all the rates equal?