Do you have a scale to weigh your items? If you don't I would suggest investing in one. There usually cheap and will prove to be invaluable. And not to mention no more running to the PO to get a quote.
Then you can check your Postal Service's website as Opalescence suggested. It most likely will ask you destination, weight and so on, all you will have to do is enter this information on there. It will then give you a quote to give your customers.
Good Luck and it is not as hard as it may seem.