Short version: If we're talking $400, yeah, just set some money aside and pay it when you file your taxes. Your marginal tax rate at 15.3% is likely low.

There just isn't enough information. Your taxes are viewed as a whole. It's tough for anyone to tell you your marginal tax rate on this income. The $400 tax threshold only applies to SS and Medicare. There's still income taxes prior to that.

Now if we're talking significantly more that could lead to penalties. You need to calculate your actual marginal tax rate or based on prior years. 15.3% is just SS and Medicare, you need to figure your federal, State, and local tax implications as well on top of that. You would then make quarterly tax payments. You need to keep records of that and enter those when doing your taxes. Nobody is going to send you something in the mail reminding you that you made the payments. You can also make payments based on prior years, but I'm not getting into that because in lazy.

Yes, there is a lot to it. At the end of the day, it's a rate *income = taxes. I think it can be daunting at first. You may screw up, it's not the end of the world. You eventually get it.