No need to send a secret agent carrying encrypted SSD disk embedded inside his skull.
Don't want to install gpg (GNU Privacy Guard) and handle all the private and public keys. Have used it many times and really do recommend - but it's not something you want to use for one-off email delivery.
Just need something "good enough" to block casual listeners in case something goes wrong (invalid email addresses, anyone) and which doesn't cause customer several day IT support issues.
Let's use Mac OS built-in application called zip and create an encrypted ZIP file archive. It is indeed known to be "seriously flawed", but a) it's there already b) it's easy to use and c) it's good enough (for this specific use case).
How to create password protected ZIP file archive in Mac OS X? Open Terminal and type:
zip -er my_archive ProjectFolderHow to open password protected ZIP file archive at customer site? Open Terminal and type:
unzip my_archive.zipJust make sure your password is long enough and contains lower-case, upper-case and special characters as well as numbers. YMMV. Send the password via SMS - do NOT send password via email from same sender account to same recipient account.