I don't know if you are using the latest zip from GitHub for 2.#.0, but that's what I am testing with. As stated, all installed items are latest from GitHub.

I turned restful back on and enabled the errors and log (BTW, I think you meant to say, "CAUTION: Each log file is at least 300KB big." versus "CAUTION: Each long file is at least 300KB big."

I added an item to the cart, went to the shopping cart, and used the estimator. I received the correct quote and no errors were shown or logged.

I then clicked on Checkout and selected Guest Checkout (OPC). I got the standard screen with no shipping or payment options. They should not show until Contact and Billing are set.

I set the Contact and Billing using the simple 5-digit Zip. Upon setting them, I see Shipping options with the Priority pre-selected. While it did list the options in the selected "Price-LowtoHigh", it might be best to also pre-select the lowest option.

At this point, there are still no errors shown or logs generated. As a test, I went back to billing and entered what my browser had stored, which was a Zip + 4 code. That resulted in a 500 error and and log files.

Since the session had the info locked in to Guest, I went back to the cart and tried Zip + 4 with the estimator and got basically the same error but a partial blank screen.

I tried the zip with no hyphen and got the same results.

My assumption (I know) is that anything other than the 5-digit zip fails. Using just a 5 digit zip works fine.

The USPS K11I accepts Zip + 4