I think the store owner explains this better than I can, so please see her email below. I am running the brand new USPS.php file, with this mornings minor change.

I have logged in as [email protected] with the password tests with a bogus United Kingdom address. This can actually be done without being logged in if you are simply viewing the estimate shipping on the shopping cart page, by selecting United Kingdom as the country. I can add items to the shopping cart up to 0.56 lb and both International First class and International priority mail options are shown in the shipping estimator (these are the only options we offer for shipping methods) and on the checkout page. At 0.58 oz, no options are shown and postage is zero, preventing a customer from completing checkout.

example used Small Head Incredible Hulk Pez early 1990s with an item weight of 0.07lb and a package tare of +10%,

I can add up to 8 of these to the shopping cart and still see both shipping options. The Cart weight shows as 0.56 lbs and I believe it is rounding this down to 0.5 lbs or 8 oz. The shipping weight shows as 0.62 lbs which would be 9.9 oz. When a 9th is added and the cart weight would go to 0.63lb with a ship weight of 0.70, the estimator breaks and shows no shipping methods.

I believe there is a two fold problem here. One part is the rounding downward of the weight increment and second is the breaking of the cart at over 8 ounces or half a pound.

The USPS calculates postage at a different rate in one ounce increments from 1 - 8 ounces, then in 4 oz increments up to 64 oz (4 lbs) for international first class mail packages. For International Priority mail, the weight increments are in 1 lb increments up to 70 lbs.