I have worked with Superior Notary Services since 2007 and they have always paid me promptly. I don't think I've EVER had to contact them about slow payment. They switched to ACH payments in the last year or so, and because of their track record with me I felt comfortable and I had no problem providing them the information. I got set up with them to do ACH a while back so I don't remember if it was an online thing or if I had to fill out a form and send it in to them. I have received numerous ACH payments from them for completed signings, they will send you an email a day or two before the deposit to let you know that it's on the way.
I understand the trepidation when working with a new company, but my experience with Superior has always been good.