All rates or charges made for any stockyard services furnished at a stockyard by a stockyard owner or market agency shall be just, reasonable, and nondiscriminatory, and any unjust, unreasonable, or discriminatory rate or charge is prohibited and declared to be unlawful: Provided, That rates and charges based upon percentages of the gross sales prices of livestock shall not be prohibited merely because they are based upon such percentages rather than on a per head basis.