Do you use coupons? If so, then you are paying a different price than those who don't. Do you fly first class or coach? Do you go to the matinee or the late show? Do you use a senior citizen's discount? All of these allow a business to charge a different price for the same good. Under the Clayton Act (1914) this might be illegal.
When business practices substantially lessen competition or tend to create a monopoly, then antitrust laws regulate the operation of the business. I doubt if any of the business practices I began this blog with less competition substantially, but it made good discussion in econ class. In 1967 Utah Pie won a case against three Los Angelos competitors using the Clayton Act. I think the real regulatory effect of the Clayton Act was to make dumping illegal.