Users need to keep in mind that page numbers specified in stamp files (i.e., StartPage) need to be Acrobat page numbers and not the page numbers as defined by AppendPDF Pro. Acrobat page numbers are the page numbers as they appear in your Adobe Acrobat window (i.e., 3 of 20).
If -f is used, page counting will begin on the cover page and AppendPDF Pro numbers will be the same as Acrobat numbers. If -f is not used, number counting is reset to 1 at the start of the body of the appended document (this will not correspond to the 3 of 20 in your Acrobat window). Therefore, be sure to refer to the page numbers as they appear in Acrobat when specifying pages in stamp files.