Same problem for me too. Even if I did upgrade my plan who knows which plan actually gives you this functionality as it doesn't tell you what you get. If I upgrade from the simple no PO plan to the next one up its another £10 per month just to put a PO number on, in a reasonable place on the document so clients don't have to go searching for it in body text, I'd sooner switch to another brand of software than pay £10+ a month just for a PO number.