The PAY NOW option circumvents the mandatory Dynamic Currency Conversion imposed by Avis for all renters paying with a credit card that is not in the same currency as the renting country.

Discussions about the DCC at this thread: Dynamic Currency Conversion Again (2017)

PAY NOW gives the option to either charge in the currency of the renting country without DCC, or using DCC to convert the amount to the currency of the issuing credit card. If PAY LATER, Avis would dynamically convert the charge amount to the currency of your credit card when you return the car. There is no ability to opt-out.