You can order free USPS boxes online from their website, or you can pick up a box from the office yourself.
Unless it's flat-rate, you will not know exactly how much the shipping will be without the buyer's zipcode. If the person tells you his/her zipcode, you can then calculate the shipping cost with the box weight/dimensions and the zipcode.
http://postcalc.usps.gov -- Unless you have a huge package that exceeds 7 feet, you'll likely choose the regular "Package" option, input the total weight, and then the zip code. You'll then have a list of various prices depending on what you choose: i.e. First class, priority mail, flat rate, etc.
Unless it's flat-rate, you will not know exactly how much the shipping will be without the buyer's zipcode. If the person tells you his/her zipcode, you can then calculate the shipping cost with the box weight/dimensions and the zipcode.
http://postcalc.usps.gov -- Unless you have a huge package that exceeds 7 feet, you'll likely choose the regular "Package" option, input the total weight, and then the zip code. You'll then have a list of various prices depending on what you choose: i.e. First class, priority mail, flat rate, etc.