I've just sold two items on eBay and used PayPal to print Royal Mail postage for them.
The first, a 2nd Class Small Parcel, cost £3.85 via PayPal
The second, a Next Day by 1pm Small Parcel, cost £9.60 including the £1 for extra compensation.
Having returned home and had time to look at the Certificate of Posting from the Post Office:
the first parcel is shown with Prepaid Amount £3.72 (That's 3.4% less than PayPal)
the second is shown with Prepaid Amount £8.15 (That's 5.2% less than PayPal if compensation is ignored)
The weights of both parcels are the same as was declared to PayPal.
I suppose the lesson to learn here is to buy postage direct from Royal Mail / Post Office
