I am not sure why they asked for a Social Security Number since it's a US only ID and not even required to be provided.
According to Avis' own FAQ the following are the requirements for valid identification:
Avis requires that each customer and additional driver meet the Avis minimum age requirement, present a valid driver's license and an acceptable driving record, and present an Avis-honored charge card or cash rental qualification at the time of rental.
The driver's license must be valid at the time of rental, and remain valid throughout the rental period. All drivers must meet Avis' requirements. Driver's licenses accepted in the U.S. are:
- Any license issued by any U.S. state, territory or possession.
- A license issued by a Canadian province.
- A license issued by a country that participated in the 1949 Geneva convention on Road Traffic or the 1943 Convention on the Regulation of Inter-American Automobile Traffic.
- A license issued by a country that has a reciprocal agreement with the U.S.
If you intend to rent outside your home country, you may also be required to present an International Driver's Permit. International Driver's Permits are valid only if presented with the original local license. An International Driver's Permit it not acceptable by itself. In addition, at the time of rental, the renter, authorized driver or additional driver may be subject to an electronic DMV check or may be required to sign a declaration attesting that they have a valid license that is not currently suspended, revoked, expired, cancelled or surrendered. A temporary driver's license is acceptable only if it has no restrictions and is valid for the entire period of the rental. Individuals with learner's permits may not operate Avis vehicles. Additionally, if you intend to rent outside of your home country, Avis may require that you present a valid passport, as secondary form of identification at the time of your check-out.
So in all likelihood the SSN request was a secondary identification for someone who entered incorrectly that you're from the US or a bug in Avis' Agent UI (yes those happen). But the Credit Card request is normal in case of toll fines, etc.