I can only answer from rules of 15 years ago when I was in Radio and TV.
We were not required to sell political ads at all. However, if we did sell ads to one candidate, we had to offer the same price to any other “qualified candidate.”
We also had to sell the ads at the “lowest unit price.” That is, we had to go back one year in our records, and figure out the lowest price we sold one spot for. That price was the price we offered political ads at. We could not inflate the price.
One station in town got stuck on that one. The owner’s wife opened a business in town, and the station sold her ads for $0.25 per minute. When campaign time rolled around, if they wanted to sell political ads, they had to be sold at the same rate.
They passed, not selling ANY political ads, which was perfectly legal, but not too lucrative.