I'm going to park in an empty reserved spot to pick something up from a porch
Are you trying to take advantage of these reserved spaces?
No, it is because I don't know where else to park
No matter how short you're going to park in a reserved spot, it is illegal and wrong. Someone who truly needs that spot can see you in that moment and have a hard time finding other spots. I'd be upset if I couldn't park in my reserved spot.
These parking spots don't serve a purpose anyway
If they're reserved then they clearly serve a purpose. Assuming otherwise is you just telling yourself what you want to hear. Can't you just get out of your car while next to the curb to grab your package?