I'm trying to use:
"//PDFX,Save.ShowSaveDialog,1"
"//PDFX,Save.FullFileName=order confirmation.pdf"
But, in the Save dialog, the suggested name is the initial filename where the PDF was generated instead of the name defined in the Save.FullFileName command.
Did I misunderstood the purpose and/or usage of this command?

The previous post was using PDFX V5, I tried this in the customer, using PDFX V3 and it works fine, the suggested filename is the specified in the Save.FullFileName command.
Well, with a couple of minor problems:
1. The filename got truncated on the first space, for example, "order confirmation.pdf" results in "order.pdf"
2. Using portuguese characters also truncates, for example, "confirmação de encomenda.pdf" results in "confirmaçã.pdf".

I think the main problem is that Save.FileFullName expects a full native file name (with a directory indicator) or an AMOS-style spec. A-Shell tries to run it through the general AMOS-to-native converter (which is probably where the truncation is occurring), but ultimately the filespec gets processed by the printer driver (which may not have the same current directory context). So it needs to be an absolute filespec.

I tried the following with both v3 and v5 and it seems to work ok:

It creates the file with the full name, spaces, and special characters.

One other problem I noticed in your example - you have an "=" instead of a "," after the Save.FullFileName. When I tried that, I got something like your results.