USPS and shipping light weight Bulky Items

i have seen variations of this question but i can't seem to fond the exact solution and would appreciate some help.

we are using Zen Cart 1.5 with USPS v15 Ratev3
max weight in Shipping/Packaging is set to 50

here is the issue

we have a 4 piece modular sleeping bag which weighs 12 pounds and when quoting shipping on 1 it falls under the max dimensions and shipping is quoted correctly @ $12.92

when 3 - 4 are order they are still under 50 pounds but the shipping quote is far too low due to the fact, putting 3 or more in 1 box would require an over-sized and what USPS calls Large Package and a different USPS rate http://postcalc.usps.com/PopUps/LargePackage.htm

here are the quoted rates from our store to illionis

1 = $12.92 / Correct
2 = $17.27 / Correct
3 = $21.58 / should be $35.50
4 = $25.90 / should be $51.80

so my question
what would be the best way to handle this and get the USPS shipping rates correct?

i have some Ideas if what i would like to do but i dont want to confuse the matter, i am just looking for the best solution.

Sean

well i guess this was a really dumb question or there is no answer

You could test for the quantity of this products_id and when >= 3 add $13-14 per qty over 2 and add that to the shipping ...

Would that adjust this shipping cost to a better rate?

might work but i dont think it will cover all scenarios, if the items were going to Hawaii, the rate would be more than in my example so the total shipping quote would still be off.

my thoughts were to find a way where

1. the item could be marked/flagged X per box
2. use the actual shipping dimensions.
3. specify the max shipping weight per item

i did find a commercial shipping module that said it would support calculating the rate by dimensions, but it appears they are not fulfilling orders at this time