That's a sore spot for me... the cheapest way (I believe is USPS Parcel Post) but it's too slow if you're concerned about getting coffee to a customer, and I don't think you can print labels online, which means a trip to the post office.
I use padded flat-rate envelopes for 1.5# and less, Regional A boxes for up to 3#, and Regional B boxes for up to 6#. The Regional A and B boxes base their rates on distance, so it can be cheaper, but if it's across several states plain old Medium FRB's are cheaper. I have one customer who always orders 8# and that requires a Large FRB. At some weight point UPS and FedEx become competitve, but I don't where that point is.
My website is set up with a $5.95 fee, and free shipping over $75. I lose money on shipping on most orders, but have my coffee prices set to absorb some of that, along with the credit card/paypal fees, etc.