If you want to be really cheap you can ship the media mail with shouldn't be more than $2-3 - it'll take about 7-10 days to go across the US
Then theres first class for about $3-4 which has a turn around time of about 4-5 days. You can add insurance, delivery confirmation (poor man's tracking) for extra dough.
Then theres Priority Mail which has default tier-pricing that I think starts at $4-6 and takes between 2-4 days to go across the US. You too can also add tracking and insurance for extra $$$. If you mail with this, USPS (Post Office) will supply you with free envelopes or small boxes to ship your stuff in.
Usually a padded mailer envelope (ones with thin bubble-wrap inside is best or maybe a thin box that you can put crumpled newspaper, popcorn peanuts or small sheets of bubble wrap are ideal.
Hope that helps.