You wrote: "I am looking for a cross-platform way to encrypt credentials I need to send to a client."
You do not define what you mean by "cross-platform." Your best solution is *not to use zip.
If "cross platform" means "Windows and Mac," then your best solution is to use Microsoft Word's encryption or Adobe Acrobat's. Both systems use AES in their most recent incarnation.
If "cross platform" means "Windows, Mac and Linux," then use Acrobat.
In either case, you will need to exchange a high-entropy encryption key for the document with your client. That is, you will need to exchange a password in order to exchange your encryption key. Of course, if you can do that, you might as well just send the credentials themselves using this out-of-band mechanism, no?
So what you really want to do is to use public key cryptography. That is, you are probably best off having your client spin up an S/MIME certificate and send your client encrypted mail using Apple Mail or Microsoft Outlook or Thunderbird or any of the other cross-platform approaches for using S/MIME. You'll find details in NIST SP800-177, "Trustworthy Email."
Other people will suggest that you use PGP. I won't, because it's too hard to use. But if you want to use PGP, give it a try!
If all of this sounds like too much work, you could go low-tech and put the credentials in an envelop, sign your name across the back of the envelope, put that in a second envelope, and send it to your client through postal mail.