US Credit Card Online Shopping in Brazil

I think that's your problem right there. Did you explain to the people at American Express when you phoned that this was going to be a purchase on installments? This is something unique to Brazil and is common to banks here and it involves the bank not charging fees on the installments, however it is not done by credit providers in other countries.

Your credit card will work just fine in any major store in Brazil for a single payment transaction, however the authorization system in other countries will probably not be set up for anything else.

I'd suggest that if your credit limit allows make the purchases in a single payment transaction. Failing having a sufficient credit limit then you should stagger the purchases, pay off one item and then purchase the next.

Firstly, did you succeed ever to make a payment online in a Brazilan shop with this card but with one payment?
All card providers consider online brazilian transactions as suspect...thanks to Brazil reputation for robed card, copied cards, etc... Using cards in shop is considered as much more secure because client need to use PIN number and be there physcally (actually I had an Anex card a few years ago and they was no need for PIN, is it still the case?).
I told you that because personally I have got problems, card blocked. I do not even more try.

Then secondly I confirm that the free credit (parcelamento) could be the issue. When using online payment in Brazil, you actually go through payment gateway  (like pageseguro, MOIP) which actually manage totally the credit for the shop.  They take a huge comission for that (could be 2-8%). Because AMEX takes already a much bigger commission than other providers like Visa or Master; so it is possible shops just do not accept free credit using AMEX card. Just ask them.

Thanks for the help. I just tried a R$10 payment on a friends credit card machine and the 1x worked without any issues where as the 3x got declined immediately. I will call the online shops soon to see if it works when they bill it in 1x

Will keep you posted!

I'm sure you'll have no problems whatever with your credit card on a single-payment transaction.

You also should know that your debit card will work for ATM withdrawals at many banks that show your network logo (Cirrus, Interac, etc.), but no foreign debit card will work for a point of sales purchase.