It never ceases to amaze me how many quibble over a couple of dollars here or there, when overall they must have considered it a good deal, or why would they have purchased in the first place.

Unfair!!  I fall into the same category of seller as the OP in absorbing little extra costs when my postage estimates turn out to be underestimated despite my best calculations.  However, as a buyer, if am quoted for postage and agree to it, with the assumption (I know, I know, don't all jump on me for ASSUMING rather than confirming, already beaten myself up about that enough)    . . . . the assumption that the buyer will be supplying item in a box or at the very least with a new padded envelope/packaging, and then it turns out that they have squished the item into a recycled envelope meaning that I've paid over four times more than the total postage cost then yes, I'm going to get narky. 

Especially since even a handling fee shouldn't be more than the cost of postage (now don't give me an example of backpacking 100kms to get to a post box or paying for petrol or the tread on your tyres). 

I guess I try to be as honest and accommodating as possible even when I lose a little here or there and I expect (perhaps naively) other sellers to do the same. 

Happily, in this case the OP is of course an honest seller.  But I would not be blocking the buyer in question, perhaps she has been overcharged before and is just trying to safeguard themself?  Has every right to do so.