That code would likely fail for numerous reasons. It uses a construction ($HTTP_GET_VARS) which is not valid in recent version of PHP; it has code specific to the orders on the site from which it was taken; it has a syntax error (missing apostrophes on an array index); and it relies on variable ($hold and $subT) that don't exist in Zen Cart. That's aside from some sloppy coding that have tax rates hard-coded, rather than looked up from tax tables. There are probably other issues too which can't be seen from this code in isolation.

How to make it work? Well that's not a question that can be answered for a snippet of code taken out of context. I recommend reposting, but stating what it is that you are trying to achieve and where.