I'd definitely put the green bars, and I liked the punch holes on that fan paper ( a drop shadow would set that off nicely). For authenticity, match the blue or faded black of the ink. I was working at a place last year that used dot matrix printers (they're still in common use on the line in restaurants) and they would print two colors, red and black with a monospace, kind of blocky font. Although the same font was used, capitalization and the two colors were used to set off varying properties of the document.
On invoices, the letterhead is usually printed directly on the paper with a logo and regular fonts (in black), but the actual inventory is printed in triplicate with a indigo ink (use a little blur to make it look right and remember the second or third copy is gray). Invoices generally had multiple copies with one page striped, then customer and merchant copies in pink and yellow. and Also consider that since dot matrix printers actually use ink, they run out of ink, and if you're doing multiple docs, maybe fade a couple of the 'older ones'. You can find some interesting images just by googling invoice + (the year you want to reproduce). Try it! Intriguing.
http://www.godfreytownsendmusic.com/my_first_real_recording_session.htm (Check out the eccentricity of the fonts on this one, and the Invoice number in red on the upper right: very typical)
Here is an image with code, but it doesn't specify paper.
You know you can use Photoshop brushes with Gimp, and some stains or smudges might be nice if you're doing the grunge thing http://www.brusheezy.com/Brushes/2168-Cup-Stains-Photoshop-Brushes.