Your FeedbackExpress plan is based on the number of emails that you send. You should try to work out about how many emails you're likely to send and then select a plan which covers that.
For example, if you have 3 campaigns and expect each to send 1 email for each order, this would be 3 emails per order. If you have 1,000 orders per month, you would need to have a plan which allows for 3,000 emails per month. So in this scenario, you would need the up to 5,000 email plan.
See packages and prices here.