You could use Trial Subscriptions which enable you to offer an upfront price on your subscription or membership. Trial subscriptions are only supported with the Authorize.net gateway. The following three fields allow you to configure trial subscriptions.

Enable Trial – Check this box if you want to enable Trial Subscriptions.

Trial Occurrences - Number of billing occurrences or payments in the trial period

Trial Amount - The amount to be charged for each payment or occurrence during a trial period. To enable a free trial enter 0 here.

Once the number of trial occurrences for the subscription is complete, the regular amount you entered in the Product Price field under Product Setup will be charged for the remaining re occurring payments.

See page 27 of the users guide
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