Couples filing jointly can buy $20k/yr, not an insignificant amount for normal people. There are ways to get more, like gifts, trusts, tax returns.
Tax overpayments, only -- the limit of $10000 of electronic bonds per recipient, from whatever source.
> The purchase amount of a gift bond counts toward the annual limit of the recipient, not the giver. So, in a calendar year, you can buy up to $10,000 in electronic bonds and up to $5,000 in paper bonds for each person you buy for.
https://treasurydirect.gov/indiv/research/indepth/ibonds/res...