In reply to Skol:
I always weigh the items and then refer to website of my preferred postal provider - usually with insurance. In the T&C I stipulate returns have to be sent the same way - things can go missing and the seller is always to blame in eBay land.
eBay now has a nasty habit of "advising" what is a suitable P&P price. A guess would be it's an aggregate of other people's pricing. They often won't accept what they deem as too pricey P&P because people were hiding costs in the postage so eBay couldn't get their cut.