Until the end of the year, one can still use credit card to purchase Saving Bonds from Saving Bonds Direct.
Currently I bonds sell for 4.66% coupon rate and EE bonds sell for 2.66% coupon rate.
I'll consider to buy some I bonds using credit card before the end of the year. The first-year yield will be at 5.66% considering the 1% cash back from credit card issuer.