What actually are managed payments? I recently bought a dozen or so cheap DVDs from various buyers and put them in the basket before going to checkout, expecting to be able to pay for them in one go; but it split them into two transactions as some sellers used 'managed payments' and others didn't. Is it anything to do with how you now sometimes pay by Paypal and select your payment card/account before confirming/finalising the purchase with eBay (like before), whereas with others, you complete the eBay part by clicking on 'Pay with PayPal' and then finish it off with PayPal? (did that description make sense?!)
It's postage AND packaging. Envelope and/or mailing, bag bubble wrap snd the trip to the PO should all be accounted for.
Honestly, I don't think it's fair to include your trip to the PO in the P&P element, as the buyer has no way of knowing whether you'll be doing a 30-mile round trip purely to dispatch their parcel and then straight home again or if you will be taking 20 customers' items in one go to the PO that's two minutes from your house, on your way to do your weekly shop at Tesco.
I can see the reasoning, and your distance from the PO and volume of sales will obviously have a bearing on whether you deem it worth eBaying something cheap (or asking a higher price) - but, although it's nowhere near the same league of CFery, I don't think it's all that different in principle from when buyers collecting something from your house expect you to reimburse them their petrol costs, because they chose to respond to a FB advert in a town group 50 miles away from where they live.
I think the P&P charge should include the postal cost and a little extra for packaging (although how many private/low-volume sellers honestly don't just re-use a box or Jiffy bag that was sent to them?) and maybe some to balance out the eBay fees, but you shouldn't expect them to pay for your travel time, petrol/bus fare, car park fee etc., just because you happen not to live near a PO or to be in a position to multitask a trip. If you live in a small village away from a sizeable town or city, you will usually have a corner shop where you can drop off parcels to be taken by a non-RM courier, anyway.
Of course, this will all be moot once the new scheme comes in where RM collect parcels directly from your home, but I bet there will still be sellers who try to blag their non-existent travel costs to the PO when upping the P&P costs they ask.