All plans (including the Free Plan) include all features.
You are not charged based on the number of contacts/subscribers you have.
Email Charges: 62,000 Free/Month. Then $1 per 10,000 Emails.
Push Notification Charges: 1 Million Free/Month. Then $1/Million Push Notifications.
Make a copy of this spreadsheet to estimate your total cost of sending emails/sms/push notifications with Send With SES.