I'm using SMTPit for mail my orders after having saved the particular layout in .pdf format with PrintoPdf.

This file is saved in a particular folder, and named "OrderN

You can do this in a single script step with the SMTP-AttachAssign function. You will need to supply the full path and filename as a parameter - but this can be done using a mix of literal text and field values if you wish.

The script step command and formula would look something like this:

Set Field ["YourGlobalResultField", "External ("SMTP-AttachAssign", "C:MailoutDocumentsOrderN*" & OrderN* & ".pdf")"]

...where the OrderN* value is being sourced directly from a field called OrderN*, but the rest of the path, before and after that are being supplied as literal text in quotes. The path can be supplied from a field value also if you prefer.

Note that the format of the path I've shown in the example is what your path might look like on a Windows computer. On Mac you'll need to separate the disk, folder and file names with colons, not backslashes.