I used to sell stuff and any postage mistake that I made I accepted as being my own stupidity. A couple of times I seriously overestimated the postage and send a refund with their parcel.
Normally the P&P charge slots in automatically on the PayPal invoice (assuming you paid that way) - so how much did you actually pay? If £8, then it's their error, I'd keep schtum about it and see if they mention it.
I would email them and let them know otherwise it will gnaw at your conscience. They will probably say forget it and thanks for being so honest. :o) most people that I have dealt with on Ebay have been very nice and honest.
Pasta, out of curiosity I have just looked at one of my recent purchases where I paid postage - the "Order details" tab is just for the item, not the P&P. Check back to your method of payment (was it through your PayPal account?) then you can verify what you actually paid.
Jomifil not everyone on eBay is honest. Had some crazy buyers this year (win and then don't pay) and sellers who try to sell me fake goods.
Also had negative left by someone who lied about their purchase. Luckily eBay removed it on appeal! Not a great year after many years almost without incident.