[magnets]

If you put a heavy weight on the end of an arm on a servo, and you set an angle, the servo will overshoot past the setpoint, and then oscillate and never settle. For powering heavy stuff, try to avoid servos. Instead, you could try to make it easier for the servo with some surgical tubing to help pull the heavy arm up, so the servo has an equal load lifting and putting down the box. Again, you will need to do the math to see if it's possible.