Instead of the -er flag, you could use the -P flag - it expects the password to follow.
Use password to encrypt zipfile entries (if any). THIS IS INSE-
CURE! Many multi-user operating systems provide ways for any
user to see the current command line of any other user; even on
stand-alone systems there is always the threat of over-the-
shoulder peeking. Storing the plaintext password as part of a
command line in an automated script is even worse. Whenever
possible, use the non-echoing, interactive prompt to enter pass-
words. (And where security is truly important, use strong
encryption such as Pretty Good Privacy instead of the relatively
weak standard encryption provided by zipfile utilities.)
Note it is insecure because the password is displayed on the command line and potentially viewable by others looking at the process tree.
If that's not acceptable, look into using the expect utility.