31 CFR §363.37

1. (a)Online purchase. Purchases of eligible Treasury securities through your TreasuryDirect account must be made online.
2. (b)Payment for savings bonds and marketable Treasury securities. You can pay for eligible savings bonds and marketable Treasury securities by either a debit from your designated account at a United States financial institution using the ACH method, or by using the redemption proceeds of your zero-percent certificate of indebtedness. You can pay for savings bonds automatically using the redemption proceeds of your payroll zero-percent certificate of indebtedness through the payroll savings plan.
3. (c)Payment for zero-percent certificate of indebtedness. You can pay for a zero-percent certificate of indebtedness by:
   1. (1)A credit from your financial institution or employer using the ACH method to your TreasuryDirect® account;
   2. (2)A debit from your designated account at a financial institution using the ACH method, limited to $1000 or less per transaction; or
   3. (3)Using the proceeds of maturing securities held in your TreasuryDirect® account.
4. (d)Payment for a payroll zero-percent certificate of indebtedness. The only method available to purchase a payroll zero-percent certificate of indebtedness is to arrange for your employer or financial institution to send a credit by the ACH method to purchase a payroll zero-percent certificate of indebtedness in your TreasuryDirect® account.