PayPal has about 12 different fee help pages for the different countries, and reading them makes me confused. Specifically about international and cross-currency payments.

If I'm a seller with a US-based PayPal account and I want everything in USD on my end, the base rate is 2.9% + $0.30 USD. If the payment is coming from outside the US, the fee is 3.9% + $0.30 USD.

And it looks like there's an extra 2.5% put on top when there's currency conversion involved. But when is that 2.5% added? Another related thing I don't get: PPCalc says that "fees are calculated in the sender's currency", see http://www.ppcalc.com/faq.html#faqlink05

Do I have to know what country the user's in and what currency he pays with to determine the fee? In other words, will I only know the exact fee afterwards when PayPal tells me what it is? Problem with that is ... we want to show our users something like "To top up your account by $100.00, you'll be paying a total of $103.30 including PayPal fees. Go ahead?"

Anyone know how this all works, or know a nice, simple page which explains it?

(Yes, I know about PayPal Sandbox, but the sandbox user profiles have limited options, and I'm not sure I'm getting all the country/currency options.)