Some of you had encountered problems where your order was defaulting to USPS Express Mail even if you had selected First Class or Priority. After investigating, it turned out that USPS had changed some of the variables used in their shipping system, and this kept our software from effectively talking to theirs when declaring shipping options. (Apparently, no one using our software knew about the change in advance, so it was kind of a surprise to everyone.)
But fear not: I found a fix for the shipping code and tested it out this morning. Everything is working fine, so all the normal shipping options are available again. Sorry for the inconvenience.