Practically, no To use the USA online store, you need to have a US credit card with a US billing address, as well as a shipping address within the USA.
Also note that the iPhone warranty is NOT international. And in Brazil, all technical support is through the authorized carriers, not Apple. You will likely find it impossible to get warranty service, should you need it,with a USA purchased product.
Does Brazil have LTE yet? The only GSM iPhone (model A1428) available in the USA supports different LTE frequencies than the GSM model (A1429) sold elsewhere. If you buy the USA available model and it does not support the LTE frequencies the Brazilian carriers roll out, then you will be out of luck for LTE data connections.