I have absolutely no idea how/what they are basing the detection on, but it most certainly isn't by extracting the file from a password protected zip, so possibly based on a it being a zip and it is password protected and possibly by the file name of the file inside the zip.
You can open a zip file and see the contents, but if it is password protected you can't extract the files, if you can't extract the file you can't scan the contents of the file. Email servers might well have AV scanners, but what they won't have is brute force password tools to crack a password, that takes just too much processing effort and simply isn't worth it.
I understand your aversion for using an email client but you really are making your life difficult. Surely your ISP has an email account for you to use My ISP (and previous ones) offer an email address and standard pop3/smtp account. Vista comes pre-installed with Windows Live Mail, etc. so it would be relatively easy to use that ISP email account.
You don't even have to directly use the email client as in the avast Program Settings, SMTP, you can input your ISP account details and that should allow you to send samples from the chest. These samples are encrypted and don't need to be zipped and password protected and I have never had a sample sent this way bounced by any email server AV scanner.
