I agree with Bernie as well, although I wouldn't rate the sellers a stupid.
Manage your shipping better, if you can't.
If you can't manage your shipping, manage your customers' expectations.
And definitely park your ego at the door.
Treat your customers like friends and they become your friends;
1. Never lie to them.
2. Never talk down to them.
3. Remember that you are their to solve their problems, not the other way around.
4. Promise less than you deliver and constantly exceed expectations.
If there's a magic formula for success, it's the above points plus hard work.
Imagination will often carry us to worlds that never were. But without it we go nowhere. Carl Sagan