If a customer wants their trial period extended when their payment has already been issued, refund the initial payment, set a reminder on your calendar for when the agreed upon extended trial period is over, on that agreed upon date, re-issue the initial payment less the amount reduced by the free trial period.

For example:

A customer has a $60/year Ninja Plan, and he's been charged his initial payment. The agreed upon trial period is extended a half a month. On the last day of the trial period, issue a payment of $60 - $2.50 (1/2 of $5/month) = $57.50.

If a customer wants their trial period extended before their initial payment has been issued, set a calendar event for the date the initial payment is scheduled to be issued so you can void or refund the initial payment. Advise the customer that they will automatically be charged the initial amount after the first 14 days, but you will void/refund it that same day. When you void/refund this transaction, send the customer an email advising you did this. Then set a reminder on your calendar for when the agreed upon extended trial period is over, on that agreed upon date, re-issue the initial payment less the amount reduced by the free trial period.