If you look at your itemised list of fees for the month so far (or your last invoice if it's on that) it'll show you exactly what the fees were on each item. It's usually broken down to fees on item price and fees on postage amount, but if it was a pick up item there won't be any fees on postage.

If you sign in to your paypal account and select the relevant transaction it'll show you the exact amount of fees paypal took from your payment.