I also live in Colorado and only got one such letter last year after a lot of online shopping. It was from B&H and was wrong, though the dollar amount wasn't ve

I also live in Colorado and only got one such letter last year after a lot of online shopping. It was from B&H and was wrong, though the dollar amount wasn't very high. I placed an order where one item was back ordered, so separate shipments came. They said tax was owed on the back ordered item twice. But, as you said, Amazon already collects tax at time of sale which was the bulk of my purchases.