I just tried using a Starwood AMEX to buy US Savings bonds. These can only be done with a maximum of $1000 face value, so if you want to do more, you need multiple transactions.

After 3 transactions went through, the 4th was rejected. A call to AMEX indicated that they electronically flagged it as "unusual transactions on-line". They were able to adjust it so that further transactions went through - but suggested that in the future a call beforehand would avoid the problem.

By the way, if you do it now (before end of day eastern time on the 27th) the bonds will be dated Feb 1.