Yes, had seen that. And from the bottom of the same link you have sent:
"For new users signing up, we offer a one-month Foundation Trial during which 5,000 emails are included for free. During the one-month trial, however, you actually can send up to 6,000 emails at no cost (due to our $1 minimum for payment processing). For example, if you send 6,000 emails or less during the trial month, you will be charged $0. However, if you send 6,001 emails, you will be charged the $1 minimum fee, and $1.00 per thousand emails sent over 6,001.
At the end of the one-month trial, the account will begin using the Foundation subscription plan which costs $35 per month (unless another subscription plan was selected during the one-month trial period)."
One can no longer sign up to Flex