[stomachmonkey]

Without looking at the plug in I can say the way it should work is hooking into USPS's API.

How well documented and how stable USPS's API / back end is will dictate how well or how poorly any plug in will work.

Shippers work on zones. They are not as granular as zip code in all aspects.

For example, if you are located in FedEx zone 1 and the product ships from zone 1 you should never pay extra for 1 day shipping as by default all zone 1 stuff will regularly deliver in 1 day regardless.

You have one product so figuring out say 6 basic ship zones and fixed cost per zone should not be a huge deal.

Mark each up 5-10% and charge a flat rate price per zone.

Some zones will cost less than what you charge, others more, end of day you should come out ahead or at worst break even. BTW, if you are not accounting for S&H either baked into shipping or broken out as a separate line item you are giving away earned income.

If you are getting upside down adjust your costs.

In my experience consumers react well to stated up front shipping costs.

The actual down to the penny cost displayed in the cart at the very last step can lead to higher cart abandonment rates.

In a nutshell, your plug in solution seems to be overkill for your current business.