Hi Bob
It's very easy to change travellers' cheques in the USA, so that is certainly an option. You'll probably need to keep your passport on you to use as ID. Another option is to use your normal bank debit card - check with your bank that your card is suitable.
I haven't used the Post Office card myself but I believe it is a pre-paid credit card.
I'm sure all of these options will work well.
Kind regards
Olly, your editor