A big problem with charging such fees is fraud - yes, some folks will actually pay the $20 per use ... but some of them won't like the advice, etc and/or are just scammers and then dispute the $20 or more they paid...
Get enough disputes and your PayPal account is suspended; may be anyways way before that if PayPal feel it's being used for sale of adult oriented services, or even merely associated with adult.
Ron
p.s. see PayPalSucks for the real scoop on PayPal
http://www.paypalsucks.com/