Frank, If the caclutor is putting 3 or 4 lamps in 1 box, increase your item size, just make sure your height for 2 items is NOT more than your largest box height but tall enough so the CS won't put 3 or 4 lamps in there..
What size did you put for the lamps?
The calculator can only go on what info we enter.
If we have item sizes entered to fit in one box, CS will figure just one box, if that item size will fit multiple in a bigger box it fits them until no more will fit and will start a new box.
It will not stop a sale, it will calculate based on your info. If it's inaccurate then your shipping cost will be inacurrate.
If CS figures putting 4 lamps in 2 of your boxes you should see double that rate especially since you don't have a larger box.
CS should start filling that 2nd or 3rd box when one box gets full.
Try tweaking your item size a little bigger.
I can't tell you what to put since I don't know what size you entered to test with.
Once last thing, are you sure you have Etsy's common box sizes "Disabled" and ONLY the 2 boxes you have entered in the custom box section?