he USPS shipping module, in particular [<== Continued Subject field from limited input area.]
add a product category, say "my shirts"
add a product characteristic, say "size"
add three values, say "small", "medium" and "large"
add a product of that category and add the "size" characteristic
set price, weight and dimensions on the product
under manage variants:
1. change the price of the "large" to a value different from that of the main product listing
2. change the weight to a value different from that of the main product listing.
The price value of the main listing is inherited by "small" and "medium" selections, and that of the "large" is handled correctly.
The USPS shipping module does not recognize the inheritance of the main product's weight value by the "small" and "medium" product variants, and it will not appear during the checkout sequence because it assumes they have zero weight.
If you force the weight values into those two variants, copying them from the main product, the plugin suddenly shows up and starts working.
The UPS plugin doesn't appear to have this problem.