I've PM'd you.
We were not planning on releasing the PO Number plugin until sometime after nopC 3.60 was released. However, If anyone else is interested in a PO Number prompt plugin for 3.50, email me at support at noptools.com. The regular price will be $35 USD (When officially released, we may initially discount it to $29, but if you want it now, it will be full price).
Below is what the config page looks like. You can choose to display the PO Number prompt in one of the widget zones indicated, and you set the HTML that you want to appear (in case you need to style the prompt text / textbox).
PONumber will display your html in a Payment Info or Payment Method widget zone.
Prompts, etc. are localized - see Languages > View string resources, and search for Contains "noptools.ponumber".
Widget zone where to render:
Payment Method Top
Payment Method Bottom
Payment Info Top
Payment Info Bottom
The {PONumberLabel} is a placeholder for the prompt. PONumberLabel can be changed in Languages > View string resources.
PO Number input html:
<p>
{PONumberLabel}: <input autocomplete='off' style='Width: 125px;' type='text' value=''>
</p>