31 CFR §363.141

1. (a)Primary and linked accounts. You may purchase a zero-percent certificate of indebtedness through one or more of the following four methods:
   1. (1)Payroll deduction, in which your employer sends funds through the ACH method to your TreasuryDirect® account;
   2. (2)Deposit by your financial institution, in which your financial institution sends funds by the ACH method to your TreasuryDirect® account on a recurring or one-time basis;
   3. (3)Through the Buy Direct function of your TreasuryDirect® account, in which you direct us to debit funds from your financial institution account to purchase a zero-percent certificate of indebtedness. This method is limited to an amount no greater than $1000 per transaction. When you use the Buy Direct function to debit funds to purchase all or a portion of a zero-percent certificate of indebtedness, you will not be permitted to schedule a redemption to your financial institution from the zero-percent certificate of indebtedness within five business days after the settlement date of the debit entry; and
   4. (4)By using the proceeds from the redemption of a savings bond, the proceeds of a maturing security, or an interest payment from a security to purchase a zero-percent certificate of indebtedness.
2. (b)Payroll savings plan. You may purchase a payroll zero-percent certificate of indebtedness for your payroll savings plan through payroll deduction, in which your employer sends funds through the ACH method to your TreasuryDirect® payroll savings plan, or through a credit using the ACH method by your financial institution, in which your financial institution sends funds by the ACH method to your TreasuryDirect® payroll savings plan.