When adding a credit card for the first time, you will need to authenticate your account by using a two-factor authentication method (2FA).
You will receive an SMS code after entering your mobile number to authenticate your account. Once authenticated, you can start processing shipments on this card.
To add your credit card, there are 2 ways:
1) In the top-right corner click on Settings > Payments > Add Card
2) When creating a shipment, at the 'Finalize & Pay' step click on Add Card under the credit cards section
Note: The mobile number does not have to be linked to your ShipTime account, but it does have to be a valid phone number (i.e. a mobile number from a phone company).