I don't think there is really much difference, I must admit I have not used PayPals system I have used Royal Mails online system and unfortunately have had problems. Even though I have a very good set of scales, on occasion the post offices scales showed a different weight. I later discovered that the PO scales are not checked or calibrated as often as they should be.So something to look out for, it will be their scale weight they will charge you for not yours even though yours might be more accurate.
Back to your question there seems little point in doing it online as you still have to take it to your local PO where it is again weighed and you are given proof of postage.
For bigger items I use Parcel Monkey who will collect same day,provided you are reasonably early to book and who's prices are very good. Better than parcel2Go who I have found to be unreliable.