bought some Xbox gold live from Brazil several months ago.
Here is what I’ve done.
1. Use a vpn. So it appears I’m from Brazil. I would use a trusted vpn since you’re entering sensitive information.
2. Redeem code in my account. I’m using a US Account.
3. Switch off vpn. Use as per normal.

you don't need any vpn now

just change your xbox region to brazil or argentina (currently the cheapest region), add a local address there for your CC (google it)

it can be a hit or miss, if got cross region error when paying, try again after few days

some says reset their xbox also will do the trick, haven't tried that tho...

how is it? does it work for you? did not work for me