How you mail your newsletter is up to you of course. A self-mailer means that the newsletter is mailed by itself, not in an envelope. To do so, plan for the space to put your return address as well as the addressee, whom you’re mailing it to. Also, leave appropriate space for the postage. Check with USPS.com to find out appropriate current guidelines for “tabbing” your newsletter. When the newsletter is folded in half or in thirds, you must have an approved way of sealing it so it doesn’t come open during mailing. Regulations may change, but currently if you have the open end of the folded newsletter at the top, put 2 sticky tabs at the top to hold the paper closed. There are circular tabs (check on the right size, but about 1 inch is correct) available at any office supply store. They can even be clear and perforated so the newsletter is easy to open and you’re not covering up any of the print. As of the date of this post the link to the USPS website (see section 3.13) that discusses this is http://pe.usps.com/archive/html/dmmarchive0514/201.htm#wp1042746
If you have a local printing company print and mail your newsletter, they can handle all this for you. And if you have about 250 copies to mail they may have discount postage rates available to pass along to you.
If you mail it yourself and you have lower quantities, just put 1st class postage or a Forever stamp on it. The points on postage apply to whether it is a self-mailer or put in an envelope.
Why envelopes? Or why not? It does cost a bit more, perhaps a few cents or up to ten cents more per newsletter. Pros: it may be viewed as more important if it comes in an envelope. Also, you can have a message printed on the envelope too. And it is easier to include an extra sheet of paper in the newsletter, perhaps a special offer for the month. Cons: just the cost.
Don’t fret over this decision. Just get your company newsletter out the door!