I'm currently working on a site where I want to use micropayments. But as adelie89 already said, providers are taking a huge part of the pie and you mostly get only a small share.
Contact Morten from
Global Access via Skype. He is very nice and helpful.
Good luck.
btw, I was looking for an adult donation solution but it seems it doesn't exist. Most donation systems are based on paypal which isn't useful for adult.