Hi, Michael,
I agree with soapbuddy. I use padded envelopes to send 1-2 bars First-Class and Priority Flat Rate boxes/envelopes for everything else. (In the U.S., First-Class mail is limited to 13 ounces; First-Class International is limited to 4 pounds.) I have 6x10 and 8.5x 12-in. bubble mailers, and you can get Priority Flat Rate boxes free from the Post Office. If you order them online, your postal carrier will bring them right to your door.
Generally, within the U.S., I charge $2.50 for one bar; for two, $3.75. Three to about 6 bars go Priority Flat Rate small box/envelope for $5.00. From there, I would use the medium and large Priority Flat Rate boxes. (The Flat Rate boxes just make life simpler.) International shipping is a little trickier to figure. Cruise around some soap shops on Etsy to get some ideas. I don't have tons of international shipping experience, but you can take a look at my shop, too. Or you could have international customers contact you first for a quote.
Get a good digital postal scale (you can find them at places like Office Depot), packge your soaps up and weigh it, then go to USPS.com to calculate postage. Round up to the next ounce if you're over or on the cusp. It is better to pay for an extra ounce than to have a package arrive to your customer with postage due. Since Priority Flat Rate has a set price, you don't have to weigh it - like to Post Office says, if it fits, it ships! (Although I believe that domestic Priority Flat Rate is limited to 70 pounds, and the weight limit for Priority International Flat Rate small boxes/envelopes is 4 pounds, and 20 pounds for the medium and large boxes.)
Some extra bubble wrap or packaging peanuts to bumper your soaps are a good idea and add little weight to your package.
You can print postage online, too. There are lots of options for this, but I usually use PayPal or USPS Online Postage.
Good luck with your business!