I strongly recommend shipping via USPS. You know the total cost up front when shipping with USPS and thus you can set your fees accordingly.
If you ship via UPS or FedEx or similar, you will pay their shipping fees up front, but you will be charged brokerage fees after the package is out of your hands. The brokerage fee can exceed the shipping cost. I will ship with UPS/FedEx only if the buyer gives me their account number so all costs get billed direct to the buyer's account, not to my account.
I'll also offer this advice, even though you didn't ask for it -- Be careful this is not a scam. It doesn't make a lot of sense for someone out of the blue to buy soap or other relatively low value items from an international seller. Most of these "cold" offers to buy are bogus.
If there's some kind of mitigating factor that explains the reason for the purchase, well, that's a different story -- you're friends with this person, or you make a really unusual product the buyer really wants, or something like that.