I see that some sites pay out monthly, and some require that you request a payout. So what happens if you never request a payout? The reason I ask, is because I neglected to monitor my earnings at FeaturePics,  and they went ahead and sent my earnings to my PayPal account, once it reached over $250.

Do any other sites do this? Are there any sites that will never send money until a payout is requested? At what limit do they automatically  send money, just to clear the books?

Not sure about the others, but iStock will never automatically send an unrequested payout. The maximum they will send at one time (due to anti-terrorism laws) is $10,000, but if you exceed that amount you can just request multiple payouts until you get all your money. 

I'm not aware of any microstock website which would send money automatically.

As far as a microstock site that you must request payout from sending money automatically, I am not aware of any either. Shutterstock does send money automatically at the end of the month.

I don't expect a fully automated processing, but also not that an employee has to manually run each payment in PP or MB.  I would expect a system generating a list, then someone would run through this list to check it and to compare with available funds and their cashflow.  I mean, if they have to pay their employees on a certain date, they have to priorize this over payouts, so if there is not enough money for all the payouts, they trim the original list and send the result through some system to PP and MB. 

Regards,
Adelaide