I have the same question too.  As per the reg, an individual is allowed to purchase up to $15000 annually.

They can use $5000 from the refund.  What happens if it's a husband and wife duo?  Is each person allowed to buy $5K from the tax refund, totaling $10K?  Or, can they only buy $2.5K each on the return and buy the remaining at USTreasury Direct website?

Thanks!