more than likely you are being a bit paranoid
if the pc is still working fine, then you obviously caused no damage. while it is true that static discharge can damage sensitive parts, simply touching the cables poses very little danger. you'd have to shock a circuit board directly to have the chance of doing any real damage and that is not 100% that you would.
you more than likely touched the case while opening it and even while reaching inside which would have grounded you and prevented any discharge. so more than likely without realizing it, you did the right thing and grounded yourself before reaching into the case.
again so long as the pc is working fine, then you have not done anything to it other than opening it.
edit: rereading i see you literally meant you touched the psu in the box. pretty much no chance you did anything to it. they are designed to handle a lot more than a bit of static and highly unlikely touching the psu would do anything at all to it.