We have a rather silly system here in the UK for our shipping rates. As you might expect, they're banded by weight. As the package weighs more, the price you pay to ship it increases. That side of things is fine.
However, we have a complicating factor: if your product exceeds certain physical dimensions, then you have to start from a different band. Large Letter in this diagram applies only to packages which don't exceed a certain physical size. If they do, then they're not Large Letters but Small Parcels, even if they don't weigh very much.
In my example, I have a book which weighs 184g. Ordinarily, you would think that the cost would be £1.40 for 2nd class postage.
Not in this case, however; once it's in an envelope, the book's width slightly exceeds the permitted dimensions to be classed as a Large Letter, and is automatically considered to be a Small Parcel, the starting rate of which is £3.10. (Even though in the official price matrix, a small parcel starts at 1kg. As I said, this is a silly system. So silly, that the firm can't even tabulate it.)
So, how can I handle this in Commerce? One situation might be to set up a new matrix for these oversized items where the categories Letter and Large Letter aren't present. The problem there is that if a consumer buys this oversized good and a normal good with the usual shipping rate, then this happens:
The poor customer is now paying two lots of £3.10, even though the combined weight of these two means that she should only expect to pay that once because the total package is under 2kg. She's not to know that there's a complicating factor here.
Does anybody have any idea how to work around this scenario within Commerce? I know that @Mark is a Brit so I'm half-hoping that he boy-geniused this with our silly system in mind when developing the app.