Go to the main post office branch in your area and they will make for you at NO cost a page layout for a #9 envelope (or whatever size you use) for you to take to your printer and have them printed. Print in quantities of 500 if you like. We buy in quantities of 2500 (a case) since it's cheaper and I know we'll use them. I use them when I send out contracts too. A pre printed #9 envelope with the all the bar coding set up to USPS specs will get to you the quickest with the least number of lost pieces. In the past 8 yrs I don't think I've had a lost piece yet, only one or two that have been slightly damaged and show up in the plastic bag. After I get the "slick" from the USPS I would take it to the printer and for a little custumization have your logo added to the upper left corner of the envelope. Mail the #9 with your bill in the #10 window envelope. It's not that expensive and it makes it easy to sort your mail to when you've got a stack of it.