USPS shipping options activated, but suddenly do not show up

I'm using zen cart v1.3.8 and I have had the USPS shipping configured and working properly for a couple years now. Recently, my option for USPS International Flat Rate Envelope stopped appearing as a shipping option even though it is activated in my shipping modules. Regular Priority Mail International is still working properly.

Out of curiosity, I activated all the USPS international shipping options to see what works, and only the Priority Mail International and Global Express Non-Doc appear when I do a test check-out. I do not want customers to have to pay $30+ to have something shipped that fits in a $12 Flat Rate Envelope.

Has anyone had a similar problem or have any advice?

What is the Ship From address and the Ship To address and the weight of the order?

Did you check the Tare settings in Shipping/Packaging where the default is to add 3 lbs to the order? Change it to 0:0 if you want to add no weight ...