Newbie: Authorize not Charge (yet) & change amount?

[SABmore]

We are building an application that will need to pre-authorize purchases, but not charge someone until a certain # of purchases have been made in total by the community (ala - Groupon).  Is AIM the only Authorize.net solution to offer the ability to authorize and later charge?

Also, is we authorize on $20, but later only actually charge $15 is this permissible?  I read some blog posts that alluded to the fact that changing the amount billed is a violation of CC regulations.  

Thanks for your time and assistance.

[TJPride]

All Authorize.net solutions allow for authorization and later capture, as far as I know. As for changing the amount billed - I wouldn't think that would apply if you're charging less than the authorized amount rather than more, and the API documentation for AIM's prior authorization and capture does specifically mention that the amount field is "required only when less than the original authorization amount". Authorize.net wouldn't include provisions for doing this if it were illegal.

[richardtuttle]

What about if the charge amount needs to increase?

[TJPride]

In that instance you'd need to create a new charge for the extra. And the only way to do that without having the customer re-enter their info is with CIM (Customer Information Manager). Unless you're a restaurant with a card present account and the extra charge is a tip?

EDIT: Also note that authorizations expire after 7 days. So if you might go beyond that, you definitely need to use CIM.