If somebody has 100 tickets to sell, then it costs them the same to man the phone lines etc, whether they are sold to 100 separate people or to 10 people in blocks of 10 at a time.
Ticket Agencies are just acting on behalf of the venues. The ticket price will go to the venue, leaving them with these charges. As I have pointed out, their costs remain the same, regardless of many separate orders there are. If any charges are based per order, then those buying just 1 or 2 tickets will bear the brunt of the increased charges.