I don't think I've had a single package delayed even a day in the last 20 or 30 orders.
When you mention the flat rate boxes .... are you orders being shipped "First Class" with shipping conf #
Or are they Priority Mail .... just not in the flat rate box ?
I always do Priority Mail with the exception of Avid, because I get his in two days everytime with First Class Postage.
It is funny that my packages from LiquidXpress with free Priority shipping always come in two days from CA to NY
But Priority from much closer places always take three days. I guess the initial flight from CA to NY saves a day.