RAMPS has a PS_ON header. The (usually) green wire in the ATX connector is grounded with that header with the M80 command is called, and then ungrounded when M81 is sent. You can put M80 and M81 and the beginning and end of your print job to control the power supply. If you go this route, you'll need to power the MEGA board either via USB, a separate dedicated power supply, or just the +5VSB from the power supply that still is available when the supply is otherwise off.
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