Exchange rates on cash purchases here in the US are never great and most now ship it from a central treasury to you or your bank branch and charge a shipping fee as well.

While people on FT sometimes whine about exchange rates on CC transactions, the reality is that the rate is typically not particularly bad (presuming that you have a CC from a "real" card issuer) and you will find that inevitably, if you have cash, you wind up with some when you are back in the US. Nothing you can really do with it other than save it for the next trip and unless there will be one, that left over cash is part of the exchange rate.