There are no-fee payment methods and fee payment methods for paying the GMAC car payment online. The no-fee methods include using online bank withdrawal with Western Union and setting up Auto Pay with a bank account, credit or debit card.
It is also free to make the car payment by mail. Some payment methods do have a small fee. For example, online bank withdrawals, when they are one-time or recurring with Western Union, require a fee. They are only free when made with ACH bank withdrawal methods. There is also a fee when paying by credit card with MoneyGram or Western Union, and when paying with cash or by phone.