I’m a Canadian citizen planing to open a little online retail store to server US customers. Do I have to open an US company or I can activate as a physical person?
Generally speaking you can run an online business as a Canadian entity (like a Canadian equivalent of sole proprietorship or any other), selling to the US residents, without the need to register in the US.
As you grow you might want to consider registering in the US if that would be advantageous from a business point of view (taxes, credit, or other considerations).
(a) You made a typo in the card number, CCV code, expiration date, name or address;
(b) Your card balance is too low;
(c) Issuing bank has declined this transaction for some other reason related to your account.