Q:

Current PayPal Fees in web API?

Does anyone know if there is a publicly available PayPal resource that would allow me to calculate transaction fees in my Magento code? 
For example, if someone buys something for $1.00, I'd like to know if $0.15 of that is a PayPal fee.
If there were a resource, like paypal.com/currentfees.xml or something that I could readout in my PHP code, I could that. Does it exist? I know there are different fees for different payment methods (e.g., AMEX) and fees could change at any time -> So it'd be good to have a dynamic way to calculate how much of Magento's order totals are really PayPal fees.
EDIT: To clarify, I'm not interested in charging customers more, just in knowing how much of my price is going to fees. For bookkeeping.
Thanks!

A:

PayPal fees are not the same across accounts or even by transaction; there are different fees for cross-border transactions, PayPal Here transactions, large-merchant discounts, et cetera.
Have a look at https://www.paypal.com/webapps/mpp/paypal-fees for more detail.
So no, unfortunately you can't pull a resource to find the current fee.  
However, PayPal does return the fees paid for each transaction in the API response.
For example, if you're integrated with PayPal Express Checkout, the DoExpressCheckoutPayment API (doc) is used to finalize a transaction. This API call returns a 'PAYMENTINFO_0_FEEAMT' parameter which contains the total fee amount paid for the transaction.  
Should you wish to retrieve this information after the transaction has been completed; PayPal also offers the GetTransactionDetails API call (doc). This API call also returns 'FEEAMT' as a parameter.
Simply submit the TRANSACTIONID and read out the FEEAMT.  
Note:  Fees are only calculated on completed transactions. You won't get a FEEAMT back for any order or authorization-ID's (e.g. those starting with O-xxxxxxxx).  

