Meghan, if a package weighs more than 13 ounces, and you want to send it thru the post office, you'd need to use either Priority Mail or Parcel Post (or Express Mail, but that's VERY expensive). Also, packages weighing more than 13 ounces must be "handed" to a postal employee. I've never had a problem just laying those packages at an empty counter window in my post office ... rather than standing in line. I just put the package down and leave. I'm sure the dozens of people standing in line wonder what the heck I'm doing, but that just gives them something to think about for the next 20 minutes or so while they're waiting! :-)
Nancy, everything I've read in the last couple of years says to charge a bit more for the item and less for the postage (or if you can offer "free" postage w/o making the selling price too high, that works too!).