There are 2 ways to purchase ads for magazine's using MagTitan:
1) The standard pay-as-you-go model. Here the advertiser creates a campaign, sets their budget and can start/stop whenever she chooses. The advertiser is billed after the verified views have occurred.
2) Pre-pay via coupon, like a Charter Advertiser Program. Here the advertiser pre-pays in exchange for a special benefit (i.e. bonus advertising.) Pre-purchased ads have a no return/cancellation policy.
If an advertiser is concerned about not being able to cancel they should either limit the amount they pre-purchase or just purchase ads in the standard pay-as-you-go method.