As far as my experience goes (selling Speaker Light Kits)...I can give you a quick rundown of USPS First Class on international orders. The box size doesn't make much of a difference (they don't even ask the box size if it isn't over 12 inches). What matters is the weight.

In January of this year the USPS really mixed up their shipping weight tiers and prices a lot for international orders. They used to go by 1 pound increments (as it is for the US destinations), but in January they changed things to be 9oz and under as the first weight tier. The next weight tier goes from 10oz - 2 lbs. So if you are buying something that weights 10oz....you might as well bulk up and get a 2lb order.

As far as my business is concerned: shipping ONE speaker light kit (which is under 1lb) WAS $17.95 and went up to $22.75. Shipping TWO speaker light kits WAS $23.95 and it is NOW $22.75. So the price on shipping 1 light kit went up a good deal, but the price on shipping 2 actually went down from what it was before the changes. That being said, the USPS also took away the discount they offered for buying through their website directly...so I now add $1 to each international order to cover the added international expenses (extra PayPal fees for currency conversion and also no free boxes from USPS since I am not shipping Priority internationally). Before, when I was offered a discount on shipping international...I always considered that discount to take care of those extra expenses. These price examples are for Australia, but are the same but $.25 lower for all other international countries except Canada. All international destinations (including Canada) are effected with this same "you can ship 2 for the same price as 1" effect for what I sell. So double up international buyers! Shipping 2 light kits is the same price as shipping 1.

For the most part, all of the boxes I use for international order are the same size. They are the cheapest boxes I could find that were a good size to ship 1 - 5 of my light kits. So if you order 1 light kit from me, there is a lot of empty space in there, but that does not effect the shipping costs at all in my circumstance as the weight for 1 (or 2) light kits still fall in that 10oz - 2lb weight tier.

Doug (SpeakerLightKits.com)

Default Med Flat price is $13.45. PayPal shipping sells for $11.95.

https://postcalc.usps.com/

I think adding the various expenses that sellers have mentioned on here to an items price is fine (I did), but only if the item is $2 - $3+. If the seller is selling small priced items: EX a $.10 screw or $.67 LED....you can't really tack the operating costs onto them. If they did, as you mentioned, it would be much worse for the buyer. You would be paying $2 - $3 per screw.

In my case, my lowest priced item is $6, so I don't feel the need to add any "handling" fee onto any orders to make even the smallest order worth my while to fill. I consider all of my costs, profit and operating costs figured into my item prices. If I were selling a screw for $.10.....I'm sure I would be adding on a per transaction handling fee to make the $.03 profit on that single screw order worth even messing with.

A seller could possibly automate things, so bigger orders did not have an added fee tacked onto the shipping. That would mainly depend on their website and checkout setup...and also if they felt like passing up that extra fee that they need for the smaller orders and are used to getting.