I'm looking to setup a business to sell personalized shirt designs
over the web. We're targeting US customers, but the actual operation
(shirt design, shipping handling, customer support) will be run from a
Central American country.

Do I need to incorporate the business in the US?
What sales tax do I charge for US customers?

Hi buffalosoldier,

The answer to both of your questions is "no". 

Do I need to incorporate the business in the US?
No. 

What sales tax do I charge for US customers?
None. There is no sales tax on international orders unless you
maintain a business in the other country. Also, customs duty charges,
if applicable, are always the responsibility of the buyer.

Regards,
hummer