You can find the payment section and payment methods under the
Billing menu:

From the
Payment section you can choose between the different payment methods as well as top up your account with them:

Under the
Threshold section you can set an
automated top up once your account reaches a minimum balance threshold:
For this you just need to activate the
Auto Payment button, set the
Threshold Amount and
the Top Up Amount. When this is set up the system will automatically deduct this amount from your preferred payment method and top up your account:

As well as adding a backup payment method which will assure your Squaretalk services will keep running even if your primary method fails:
You can also transfer funds between accounts if you have multiple by selecting "Transfer funds" from the Payment section:
Next select your from and to account from the drop down options:

Enter the amount and select the currency after which click "Submit" to complete the transfer: