I am a commissioned sales position plus base salary. Our commission's are based on a percentage of profit margin. The product is the same price to the customer regardless of the payment method that they use. However, if the customer uses an American Express credit card, the 3% surcharge that AX charges companies to use their card is deducted from the profit margin for those orders and the sales representatives are then paid on the lesser dollar value. If the customer uses any other credit card, or a net terms account the sales representative is paid at the full profit margin rate.

Is this legal? Should the sales force be paying for what is essentially a cost of doing business for the company?

Basing commission on the actual profit to the company is legal.

Agreed. I am in CA which has generally the most favorable commission rules to employees in the country, and even CA would have no problem with such a rule. OH is not my state, but it is generally considered to be on the "little or nothing in the way of state labor law" side of things.