Thank you for your quick response. 

Just to confirm, a large company like Xerox or Canon that receives hundreds of these emails a day should open this email in regard to which invoice the sent check is going to pay for before it arrives? If that's the case, it seems like it would be hard for large companies to keep track.

Am I correct in assuming the only workaround is retyping the invoice numbers into the note section? If this is correct, wouldn't it be easier for me to just use my banks bill pay service for free? Remind me how I suggest a change to your developers.