I have the same problem with USPS. Two of my categories of products (CatA & CatB) fit into an International Large Envelope. A third category of products (CatC) is too thick to fit in that class. It has to go in a box as a First Class Package. The problem is that customers will choose the Large Envelope option leaving me to pay the difference in postage from Envelope to Parcel, which is sometimes double what they pay.

Adding weight or "dimensional weight" to the item description is a hack, and backfires with other calculations and draws complaints of high shipping costs, and abandoned carts.

My box is 3oz and is entered in the tare weight, but that skews the estimator for the Large Envelope rate. Adding attribute flat fees also drives up the shipping costs for customers that buy several items.

It seems like a really simple solution to feed dimensions of products or category preferences to the USPS so it can negate the Large Envelope option if the dimensions are over a half inch thick.

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