Confidence is a tough one. Some people seem to have it and some just don't. Communication skills can be built over time; however, for a lot of people it's a lifelong challenge. Being confident enough to talk to customers doesn't mean that your staff should sit down with them and have a four-hour conversation about the effects of global warming or tell risque jokes. It simply means that they are confident enough to interact with customers on the subject of their needs or wants in purchasing a good or service.
If any of your staff have difficulties communicating with customers, consider enrolling them in a personal development or public speaking course. There are a lot of courses offered dealing with areas such as starting a conversation, overcoming awkward moments, thinking of topics to talk about, use of voice inflection, and many more new and innovative areas.
Another interesting point to note here is that many people feel that they aren't confident when it comes to talking to people, but to other people observing them they appear to be incredibly confident and very effective communicators. I always remember an old friend of mine passing on some words of wisdom: "What you perceive as your greatest weakness other people perceive as your greatest strength."