OK, what's the protocol?

Your underestimation, and eating of shipping costs in the past, does not
mean diddly to your buyer who is being asked to pay more than he/she
should. IMO, shipping costs should also cover the cost of shipping
materials, i.e. a Handling fee. That's the way every retail merchant
charges for shipping and handling, and buyers of used gear should
welcome paying a little extra for excellent quality packing including
double boxing and the proper packing materials. So, be sure to include
an appropriate amount to cover any additional packaging expenses you
incur.

Bottom line, if you feel any guilt about not refunding a shipping overage
(and I'm not talking about $5 here, unless it's on a small item like tubes),
then you should refund the dough.

Anyway, just one man's opinion.

I generally quote shipping and handling as a charge on top of the price of the item. I normally include enough extra $ (usually $3-5 depending on weight and packaging complexity) to make sure I don't end up eating FEDEX charges; but nowhere close to a fair cost for my labor, gas, and packaging.

If the deal was negotiated as covering only exact shipper charges, then I would refund that, but only in that case which I would avoid because it's just not worth the effort IMO.

Please, I don't really have time to split hairs on shipping costs.

"Your underestimation, and eating of shipping costs in the past, does not mean diddly to your buyer who is being asked to pay more than he/she
should."

No, but, it will influence my personal shipping policy formulations. If I'm willing to refund, I should be willing to ask FOR a refund. 'Not sure I'm ready to do that. Also, Im only asking them to pay what we both agreed to.

I think if it is a material miscalculation it should be refunded. Otherwise, I think it all washes out.

I think we're essentially on the same page.

It's easier in the long run to calculate the shipping costs up front and add $5 or $10 for materials. It's not that diffcult to find the manufacturer's shipping weight and calculate cost.

If one ballparks these figures, then one lives by the estimation and moves on.

I think it greatly depends on the overage. For instance you quote 89.00 and the shipping is 20.00. If you ship ups or fedex then the client will not ever know so your final shipping price would be up to you, then your will make a determination of how you treat your customers.

But If you spent 3 hours packing the article and 20.00 for packing material and 1 hour driving to send the package away then make sure you dont cheat yourself. You know whats fair just let that be your guide.