Certified Mail is limited to 13 ounces. That's more than 60 pieces of traditional office/copy paper, so most people don't need to worry, but it's a limit all the same. If you are building a job on the LetterStream website, we'll point out if you go over that limit and recommend that you print your documents on both sides of each sheet of paper (duplex/double-sided print). If you still have more than 13 ounces, you might try scaling the font size down on your letter to reduce the number of pages. Another way that works even with hundreds of pages is to break your letter up into a few different letters, each with about 60 pages in it. We recommend this, as there isn't really an equivalent to Certified Mail in any other postal products.