So many good points. As a craftsman I have gotten behind when shipping stuff and I have had vendors slow to get stuff to me. I have found that the biggest thing is honesty and communication. I once had a resin supplier actually tell me they lost my order. We had a good laugh and they shipped it that afternoon. When I have gotten behind I tell folks and we can usually make things work.
I use click and ship through USPS, if you regularly sell stuff getting set up that way makes a lot of sense. I am on a rural route and my carrier regularly picks up packages for me.