Two-way postage is included, you just pop your film in and mail it. These companies will only develop their own film. The quality is excellent. You can get mailers for slide film, too. You know about these.
Prices are moderate, quality is average.
Look for a store where they color-balance each individual shot. This will be the place where professional photographers go. You pay only for the shots that come out. I once opened up the back of my camera after shooting my friend's wedding. A store like this saved all but 3 of my shots.