Here are links for comparing rates from the 3 main options for shipping small packages:
US Postal Service:
Carton and Packing Material Costs:
A quick and easy method for getting costs for a variety of packing materials: http://www.uline.com/. If you plan to buy in bulk (>100 units) it may be worth talking to a local supplier for better rates.
Shipping something larger than 100 or so lbs.? Check out http://www.freightquote.com/ to get a shipping rate and http://www.eroutingguide.com/ to help with the shipping paperwork.