Even if you are relying mainly on credit or debit cards as your source of funds, it’s not a bad idea to take some cash and traveler’s checks in reserve. You can change these in banks or at exchange offices (casas de câmbio) in and around João Pessoa. Banks have slower, more bureaucratic procedures but on the whole give better exchange rates (an exception being Banco do Brasil which charges R$40 commission for every traveler’s check transaction).
You’ll usually get a 1% or 2% better exchange rate for cash than for traveler’s checks. Checks, of course, have the advantage of being replaceable if lost or stolen.
Both cash and traveler’s checks should be either in US dollars or euros, and Amex is easily the most recognized traveler’s check. Thomas Cook, Barclays and Citibank traveler’s checks are less widely accepted, but you should be able to cash them in large cities.