I sent a private message to you about how we ship our coffee and items Flat Rate USPS. It may give you ideas.
You are running into the same problem we did. You want to use a flat rate (if it fits it ships) shipping option with items which vary in size and weight.
We use ZoneBased shipping and don't go to USPS for costs because they can't tell you how many of a product fits in their boxes, etc.
We chose our most commonly shipped products and determined how many would fit into each type of shipping box we had. The uncommon items were "adjusted" to fit. You want to get right, the cost to ship your largest and/or most popular items and perhaps find a different way to handle shipping cost for small items (maybe include a ballpark shipping cost). I include $1.50 to the price of calendars because if they order them by themselves, it covers the cost of first Class shipping. If they order the calendar with a bag of coffee, I add an extra calendar or other item to offset that they paid an extra cost that was hidden in the price.
We use the WEIGHT configuration in the cart more to indicate volume. If you can fit 5 one pound bottles in a box, or you could fit 25 envelopes of items in the same size box, we would internally configure the envelopes of product to weigh .2 pound (1/5 of a pound each). Then the cart is told that 5 pounds fits in a box that costs $5.05 and that is what the customer would be charged for 5 bottles or 25 packets or 3 bottles and 15 packets, etc.
We created spreadsheets that help decide how the Zone rates are configured and the spreadsheet does almost all of the work. You just enter the result in your shipping module.
Here are files I created for customers:
http://planet-aloha.com/files/Explanation.doc
http://planet-aloha.com/files/USA-Shipping.xls
http://planet-aloha.com/files/INTL-Shipping.xls
http://planet-aloha.com/files/Canada-Shipping.xls
In the example files, the customer is using a regular flat-rate envelope, not the new padded ones. When USPS prices change, just update the 3 costs at the top and reenter the string of charges to the Zone area in the shipping module. The spreadsheet is also a reminder of what fits in what size container. Odd items are "standardized" to parts of a pound per the explanation.
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