Thanks for your prompt answer RixStix.

The real problem (rather than speaking abstractly) is that I have a matchbook sized product. Several fit in a First Class Large Envelope and cost $6.55 to most of the world. I have an add-on product for that item which is 3"x3"x2" and glass. It cannot go in an envelope. However, my clients complain if they are charged the First Class Package rate (about $13) if they only order the matchbook sized product without the add-on that only fits in a box.

The USPS international rate for Large Envelope is half that of a box. A box that is 4 ounces is $3 cheaper than a box that is 4.1 ounces. A box to Australia with an item from the category that doesn't fit in a Large Envelope costs me around $13 plus box/packing/tape/etc. while the customer pays $6.55 for Large Envelope because that option is available.

Seems like it would be pretty easy to code something that negates the Large Envelope option when a customer puts the item in their cart, rather than adding tare weight and incorrect additional product weight, or adding expense to the customer by shipping a $9 quarter ounce item in a $13 box. I was reading a thread about golf shoes where there was an exception for that product getting First Class shipping. My custom code, if I knew how and where, would split that distinction into First Class Large Envelope and First Class Package.