can't upload more than one file per session on zmodem

I am working on a website that I have to upload to a unix server. I know next to nothing about unix and I have tried to use Koalaterm and Absolute Telnet to upload files via zmodem. It will upload the first file when I type "sz myfile", however to upload any files beyond that I have to exit and start a new session. Can anyone help me with this? I am doing this site free for a non-profit agency that knows nothing about how it works.

Thanks in advance

Why don't you try ftp?