The responses here are inaccurate or incomplete, and Turbotax (I use the desktop version) is completely unhelpful. First of all, you MAY purchase i-bonds with your tax refund even if you have already reached the $10,000 on a direct purchase. Second, Turbotax provides no help that I can find on this, but you need to go into "Forms" and search for Form 8888 and fill that out. About halfway down is a section to have your refund used to purchase I-bonds. The form as provided by Turbotax is clunky, but you basically have to first (higher up) fill in how much of your refund you want paid to your regular direct deposit account, and then fill in the I-bond section with the amount you want sent to Treasury Direct for that purchase. The next section provides for purchase of a paper bond, but as far as I can tell, that is not necessary; you can purchase a digital bond with your tax refund.
