Do not pay. Nothing will happen. The fine will increase to around double that, and then they will either just cancel it or expedite it to a collection agency.
For that small amount, the costs grossly outweigh the benefits. Going after you financially in your home country is very very unrealistic – no matter what various morally high horse internet users will tell you.
You will be able to go back into the US without any problems. If you are stopped in California you might end up paying the fine.
Immigration is NOT linked with minor traffic offenses. Unless there is a warrant out for you. Then you could have a problem.
From the official US Visa page:
Travelers with minor traffic offenses, which did not result in an
arrest and/or conviction for the offense, may travel visa free,
provided they are otherwise qualified. If the traffic offense occurred
while you were in the United States and you have an outstanding fine
against you or you did not attend your court hearing, it is possible
there may be a warrant out for your arrest and you will experience
problems when applying for admission into the United States.
Therefore, you should resolve the issue before travelling by
contacting the court where you were to appear.
However, I am willing to bet that noone has ever had a warrant out on their arrest for a single parking fine.
Pay the fine. You will be able to go back into the US without any problems.