See Leslie Bonacum's posting on this issue. As she makes clear, some companies do not want their sales reps to be involved in solving billing problems, etc. They have go to great lengths to develop a customer service program which will be better equipped to deal with such problems. It really depends on the company. In a smaller company, a sales rep may be able to take on customer service issues.
Larger organizations tend to specialize services.
Sales reps are expected to cover a lot of territory, and may not have the time or training to effectively deal with an account problem. The best thing to do is ask questions. Find out exactly what your sales rep is able to do for you and what the protocol is within the company. Ask the rep and a superviser at customer service - some sales reps may be eager to tell you what you want to hear, which may not be exactly kosher. The information contained in this electronic message may be legally privileged and confidential under applicable law, and is intended only for the use of the individual or entity named above.