I've implemented $3.50 flat rate shipping in the US. This is the lowest USPS First Class Package rate - basically what it costs to mail a set of steel guitar strings. I've also eliminated the "Free Shipping over $50" option. These changes will make everything simpler for us on the bookkeeping side, and will save our customers money on most orders.
You can still choose to pay extra for USPS Priority Mail if you need your order faster.