For example: sending $100 at base US rates (2.9%, $0.30):
Fee is $0.30 + $2.8913 = $3.1913 (applying the % after the flat rate)
Fee is $2.90 + $0.30 = $3.20 (applying the % to the $ sent)
According to PayPal’s What are the fees for PayPal accounts? page, the percentage does not include the flat fee:
The fee in the U.S. is 2.9% plus $0.30 USD of the amount you send. For example, if you send $100.00 USD by credit card, the fee would be $3.20 USD ($2.90 + $0.30).