Uncle is selling US Savings Bonds online which you can charge to your credit card.
The inflation indexed "I Bond" is presently yielding 7.49% plus the interest tax free (on a state level only). I bought $15,000 worth using my NEA card (which is their level for an air tic)(I don't know if the tic is taxable). The US web site states that buying a bond will be treated as a PURCHASE. www.savingsbonds.gov