Flat Rate Box Pricing

Hi -

Has anyone had success in writing a rule  or found a work around for USPS flat rate boxes where dimensions can become an issue.

What we experience is that if someone purchases 20 widgets that only weigh an ounce, the USPS flat rate box is less than UPS. That is great unless the customer buys 21 widgets, then it will no longer fit but the weight is still within the rule guidelines so we are stuck covering additional freight charges.

Any input would be much appreciated.