No Shipping Options Available - OpenCart 1.5.6

I'm attempting to set up an OpenCart store for a client.

I'm getting the following error on the shipping page.

"Warning: No Shipping options are available. Please contact us for assistance!"

Research suggests that this error happens when there is a mismatch between the weight-class for the store and for the plugin, or something similar.

I've tried every combination of configuration settings that I can think of without result.

I'm not familiar enough with OpenCart to debug this issue. Where do I need to start looking?

My troubleshooting procedure:

• The store weight UOM had been set to ounces.
• The Fedex plugin doesn't support ounces as a weight UOM.

Nothing works.

• The store weight UOM was changed to LBS.

Nothing works.

• The package size was set to FedEx 10 KG Box
• The Fedex plugin can't convert from Lbs to KG on the fly.
• The package size was changed to "Fedex Box", without a weight class

Some products now working, all shipping estimates are WAY high.

• When changing the default UOM for the store, no existing weights are converted in to the new units, although any weights stored without a unit are now read as being in the new unit.
• This meant that the fedex system was trying to pull prices for items that "weighed" hundred of ounces (which it should have been able to do, even though those weights were incorrect.)
• I updated the weights on all products to be in line with their unit of measure

At this point, the plugin was working for most, but not all, products, with reasonable accuracy.

• I changed the plugin settings from List Rate to Account rate.

Now everything works.

To simplify - The fedex shipping plugin in opencart 1.5.6 will only work if:

• All the products in the system have their weights stored in the same UOM.
• That UOM is either pounds or kilograms (not ounces!)
• A geozone is set, and a zip code is supplied(zip codes are important!)
• The package size matches the unit of measure for the weight (no mixing kilograms and pounds!)
• The product weights are actually correct
• The account in question has a rate for a package of the indicated size

Hopefully someone else will find this helpful.