I need to restrict shipments via the USPS shipping module to less than 8 ounces per package.

The reason for this request has to do with some restricted (hazardous) items I ship. The postal service will only accept one bottle (flammable liquid container) per package. Since this product is only available in four ounce bottles that means I can only ship one four ounce bottle per package. The only USPS service option I will be using for my store is Parcel Post. All other products I sell in this store exceed the volume limits of/and are prohibited in the postal service.

Simply stated:
Maximum weight per package, USPS, is 8 ounces (this will be the product weight plus tare weight).
No effect on other shipping modules (UPS, FEDEX)

There are a couple of approaches I have considered:

  1. Add functionality to /includes/modules/shipping/usps.php that would compute how many packages would be needed to ship x number of bottles and then multiply the quote by that number (similar to admin-> configuration-> Shipping/Packaging-> "Enter the Maximum Package Weight you will ship". This coding is likely to be extensive and the benefit of such resolution may not warrant that amount of work. For orders of 2 or more bottles, UPS pricing is more competitive. Having said that, this approach would take care of multi bottle orders to Hawaii, Puerto Rico and Alaska where USPS is the only option. (UPS Ground Service flies to these areas and air transport of these products is prohibited.)
  2. A simpler method would be to simply place a limit on the maximum weight (8 ounces) to be shipped via the usps module. If a person orders more than 1 bottle, the weight will exceed this limit and the USPS quote will not be displayed, however UPS will still be displayed.
Currently I offer only UPS ground service for all products. Providing for USPS Parcel service for single bottle orders will lower the shipping costs for these customers.

I'm hoping someone can help me figure where to place the code within the usps.php file. This file configures admin, posts configurations in the mysql database, obtains quotes from usps and probably a number of other functions. I can come up with an if..else statement for approach #2 above, but I get lost within the usps.php file and can't figure where to place it.

Any help would be appreciated.