Yes, I've had the problem. Still haven't solved it. My stuff is long (dimensional charge), mixed with small objects, mixed with countries all over the world, half of which have a shorter length limit tat many of the products can't go to.
I've posted about it myself: [webmasterworld.com...]
I considered doing a 'shipping included' model and adding the cost in, then instead of calculating on an item to item basis... just try to calculate year to year. I figure as long as the cost is covered for the year, it all works out in the wash. However, I still haven't found the time of guts to implement it.
I'm sure there's coding gurus here that could come up with a solution, but I just don't have the patience for that kind of thing now. I'd have to hire somebody, which I really don't want to do.