Well, I personally don't offer trial for hosting services, I do offer for software but not for hosting services.
Anyway, if you plan on doing it, it's very easy, either with 2checkout or authorize.net
With autorize.net a easy way to do it is to simple authorize the sale but not capture it and when/if the customer signs up for the sale you convert the authorized sale and capture it.
I've signed up in the past for 2checkout account (twice) but wasn't a good experience, nevertheless, if I recall correct when you are creating the products/buttons for the order you have an option there for the trial period or when the credit card will be charged so simple use it.
But will the authorization still be good 15 days later? There is definitely some kind of time limit on it (I've heard anywhere from 48 hours to 2 weeks, depending on the type of merchant account you have). You may need to authorize the card again before you can settle the transaction.