We have here one of the early issues in grade school grammar, the difference between may and can. May means permission; can means ability. If you say “How may I help you?” or “May I help you?” you are asking permission to give help. In the first question, however, you are also asking the respondent to give you direction on how to help. If, on the other hand, you say “How can I help you?” you are asking the respondent to assess just what abilities you have to help in the first place. They are both three-letter words, but may and can have a lot of baggage attached.