Many people have no idea about some truths of Magento USPS:
1. The First Class International Package/Parcel service has the weight limit of 4 pounds. So if you are using other units on Magento for the display of weight on the front end, please add another text field for that. Because you need to keep the actual product weight field for the real weight that would be calculated by USPS, with the unit of pound.
2. The concept of "weight unit", in Magento, before you enable any shipping extension in Shipping Methods, means nothing. No unit is actually assigned to the field of "weight" on product management page. But, once you enable a shipping method, like USPS, the shipping method will take the numbers in the product weight field with its own unit (which is pound in USPS services).
3. If any of the order total weight is over the limits of the shipping methods you allowed in USPS Magento backend, then the methods would not display on the one-page check out to the buyers. For instance, if a buyer (deliver address is not in US) purchases 3 products, totally 8 pounds, then on the check out page the First Class International Package quoted price will not appear, instead, other methods like Priority Mail International would appear (if you selected them as allowed methods in the USPS Magento backend).
4. Sometimes when you see problems that no shipping quote can be displayed, besides for the possible over weight problems, you should make sure that the current USPS account you are using is only used on the website you are working on.
If you have any related questions about Magento shipping, please feel free to email us at email@example.com.