Get a home scale so you can weigh what you're sending out.  Unless you use a flat-rate box or envelope, what you send costs a certain amount based on how much it weighs.  Using PayPal or the USPS online service, you can print a postage-paid label reflecting the item's weight.  Parcel post is the Postal Service's standard package sending service and may take up to a week or so for deliver; Priority Mail is 2-3 days.