When a member is removed during the billing period, we will not charge you the full price for this user. Because you have already paid a full price, we'll add pro-rated amount of credits to your account as a refund for the time you've pre-paid but did not use.
You are in the Team plan and have 3 users in your team. This means you are paying $18 x 3 = $54 monthly.
When you remove a single member exactly in the middle of your billing period, you will be refunded half of price for a single member: $18 / 2 = $9 in credits
This means you will get a $9 discount on your next month's payment.