As it turns out, that $1 bill payment was not the problem, since another one of my accounts got locked today. I think it was for loading BB, as that is the only thing they have in common. I don't really know why I loaded bluebird, as I could have just bought more MOs, but I was annoyed because my visa gift cards from staples had not come in the mail.

So, it very well could be that doing one $3k transaction at Walmart is fine, but doing 4 purchases of $500 each at walmart within a 3 minute window will lock the account.