31 C.F.R. § 363.59
A payroll savings plan is an automatic method of purchasing savings bonds. (See the definition in § 363.6 .) You may open your payroll savings plan by selecting an amount, series, and registration for your savings bond purchases using functionality in your TreasuryDirect® account. Each bond purchase must be in a minimum amount of $25 with additional one-cent increments above that amount, up to a maximum amount of $5000, in any one transaction. The series may be either a Series EE or Series I savings bond. The registration may be any authorized form of registration for an electronic savings bond. You must also initiate a request to your employer or your financial institution to send credits on a recurring basis to your payroll savings plan through the ACH method to purchase a payroll zero-percent certificate of indebtedness. (See Subpart D for more information about a payroll zero-percent certificate of indebtedness.) When you have accumulated a sufficient amount of payroll zero-percent certificate of indebtedness to purchase a savings bond in the amount, series, and registration that you selected, the TreasuryDirect® system will automatically redeem your payroll zero-percent certificate of indebtedness and purchase your selected savings bond.
31 C.F.R. §363.59