Ten seconds? Using such ludicrous artistic licence in trying to make your point makes me very tempted to ignore you. As it happens I'm frequently in town and would probably just drop it off. However I wouldn't judge anyone that felt otherwise as selfish or being an arse.
You contend ten seconds is the product of "ludicrous artistic licence", then concede that for you personally you would indeed "just drop it off" (i.e. the cost to you would be minimal)?
The ten second figure was hyperbolic, make no mistake about it. It was intended to highlight the absurdity of the statements made by countless contributors to this thread that presuppose the recipient of duplicate goods should not have to undergo any effort whatsoever to return them to their owner. I don't believe, nor did I assert, that these people were selfish.
Your point about 'Due to an honest mistake on the part of a retailer' is moot. If I make an 'honest mistake' (as opposed to a dishonest mistake, isn't that an oxymoron?

) that does not mean that the person subject to my stupidity should be forced to trek to the post office to return it.
The fact that this was a mistake is entirely relevant. There is legislation in place (i.e. the Distance Selling Regulations) to prevent retailers from sending unsolicited items then demanding payment, or maliciously doing so such that the recipient incurs costs in returning them, but this does not apply in the case of an honest mistake.
No, I made the mistake and should be forced to rectify it *entirely* at my cost, both in terms of time and money.
That would be the ideal situation, but there are utopian ideals and then there is what we may consider reasonable. In this case the retailer
is incurring a cost by providing the recipient with a freepost address (and/or prepaid envelope) to return the item.
The point of contention appears to be the fact that the recipient has to travel to their local Post Office branch to gain proof of postage. You consider this unacceptable, whereas I think it quite reasonable. Without proof of postage the recipient may not lodge an insurance claim with Royal Mail in the event the item is lost in transit; perhaps more importantly, the retailer has no proof the item has been sent at all, and the recipient may fraudulently claim it has when it has not.
It is my view that this is not an unreasonable amount of effort, that although the retailer would ideally compensate the recipient for this effort, the cost of doing so would undoubtedly be greater than the appropriate amount of compensation, and that the reasonableness of this request is compounded by the fact that, regardless of the retailer's mistake, the recipient continued possession of the item incurs a cost for the retailer.
Ultimately, it is wrong to knowingly maintain possession of property that isn't your own when returning it to its rightful owner requires only a minimal level of effort. An analogy would be finding a person's wallet on the street, that they have dropped by accident. An honest mistake leaving you in possession of another person's property. The only practical course of action to reacquaint the owner with their property is for you to incur the minimal cost and effort involved in handing the wallet over to the police.
According to your argument you are surely committed to the view that this person should not hand the wallet over to the police, or make any effort whatsoever to return the wallet to its rightful owner?