Finally, I made this 8-year-old DHL code work for my site. Yeah. If you like to try my code, please use the link below:

https://github.com/njcyx/dhl_xml_zc.git

The original code link is the following (@bislewl):

https://www.zen-cart.com/downloads.php?do=file&id=1910

I'm based in US and I'm only use DHL express for export. I only tested my code in zc 1.5.7d and php 7.4.

It is a really long long story for the debugging... I never thought I could make it work.

Special thanks to upsoauth plug-in from @lat9 and fedexwebservices plug-in from @Numinix.