If you've operated an online store, what have you learned that wouldn't be immediately obvious?

Anything counter-intuitive?

How do you handle payment vs. shipping?

How necessary is it to assume customers don't pay attention to instructions?

What would be your top five suggestions to a newbie?

I have a few, but don't use a shopping cart. All my sales are custom high end stuff. If you are going to have an online business the most important thing is optimize your site for the customers. Search Engine Optimization is very important. Use key "phrases," not keywords.

I take PayPal and my shipping costs are close to the actual costs.