I'm going to hold on to my neighbors' deliveries on their behalf if they sit out on the porch too long
That may be real nice of you. Do you have an arrangement with your neighbors?
No it just bugs me, so I'm doing this on purpose to annoy them.
If you already have an arrangement, then it is good to protect someone else's property if necessary, but otherwise, you shouldn't take things that don't belong to you.  Your neighbors will think that their deliveries have been stolen if they find them missing.
Why should I be considerate?
I'm just saying that it's really important to ask your neighbors beforehand to be a guardian for their materials. How would they feel if they didn't know what you were doing, they might feel like it was stolen. Why not ask them first if they are okay with it?