I am having the exact same problem, with estimates and with checkout.

And I noticed on the USPS website last night that they are bringing out a new large flat rate box as of March 3. I wonder if this has anything to do with it.

I remember last year with the postage fee changes there was a lot of problems and a lot of sites went down.

I have turn USPS off at the moment and it has fixed the problem but I have a lot of orders that are international and UPS is just too expensive so we will most likely loose sales, which is not good.

Has anyone go a patch for the USPS shipping module.