I would suggest using a utility like winzip as follows:
Create a new Zip file
Add the items you need to protect to the file
Select the encrypt option and supply and confirm a password.
The longer and more momplex a password you use the more secure the encrypted files will be. You should send the password seperate from the file to reduce the risk of problems if the zip file falls in to the wrong hands.
This is only one possible solution and it wont stop anyone you sell to from decrypting the file and giving copies to their mates.