Just don't forget to mention there's liquid inside it when you take it to the post office to ship it off. Be sure to pack it somehow where the bottles don't burst if they get dropped, etc.
A priority mail flat rate box would be the best choice as some others stated, but they might give you cheaper options depending on the country. (For example, you can send things using a slower method to Canada to save about 50%) I would expect to pay around $25 to ship it. When you fill out the form to send it, mark it as a gift so your friend doesn't get charged customs.