Although it may appear that a free trial member is getting billed immediately, they are NOT being charged.
For a free trial, the card is testing (pinged) with an authorization only. This validates that the card is ok.
They may get a receipt from your merchant account indicating an "Authorization Only" for the amount that eventually will be charged if they stay as a member.
That ensures the card is valid and COULD charge the eventual amount if it had to at the time of the order.
The card is not actually charged and this "Authorization Only" transaction expires after 30 days in Authorize.net. Depending on their banking system, they may see the trial amount as a possible "Pending Transaction". This means their card is aware of the initial ping but it wasn't charged. Different banking systems handle this differently on whether they report that or not.
These "Authorize Only" transactions should be allowed to expire naturally as they are no longer needed. Again that happens without any intervention on your behalf.
Then when the free trial is over (30, 90, 2000 days) in the future, MemberGate sends the actual transaction (Authorize AND Capture) to Authorize.net. That is what actually charges the card of the card holder. Your card holder is not charged anything prior to then.
That bears repeating: You should NEVER go in and force a capture of the amount in your merchant account. The software handles all of this automatically.
While there is no limit on the length of the trial, the longer you make it, the more likely that some problem with the card may come up in the meantime (expire, over balance, cancelled etc).
The first instance (Authorization only) should not ever be touched. It will naturally expire on its own. That's what it needs to do.
The second instance (Authorization and Capture) is the actual amount being charged against the card.