Many of the membership plans need one time fee. In order to facilitate this, there is a plan level setting for a one time fee.
In the plan setting , there is an one time fee. Just put the amount you want to charge for that particular plan apart from the Plan cost. This way, when the person joins for the first time, he/she will pay plan cost plus one time fee. But those who renew their membership, this one time cost will not be charged.