I suspect it has something to do with the plugin timing out while waiting for a response from USPS.
Is it possible to move the timeout length from a hard-coded number to a software adjustable (user input) on the plugin configuration screen?
This has been occurring since at least 4.0 and is continual now on 4.40.4. My customers would like to have a choice of which carrier to use, and the low timeout value is limiting the values to only UPS.
If the customer continually hits either refresh or the Shipping Options tab button, eventually USPS will respond, but it can take up to ten refresh cycles to get to it.