If there's already a thread for this, someone please refer me to it and mods can lock this up.
Just use your judgment,
if it's same state and a small item just use priority or parcel
If it's heavy and across the US, just put it in a flat rate box/envelope
Honestly the difference isn't significant so it doesn't even really matter, maybe a couple cents to a dollar or two unless it's something bigger like a pair of shoes.