At the time of purchase, a merchant "pre-authorizes" the transaction to your card. This reserves the estimated dollar amount of the purchase. Typically, within 24 hours of the pre-authorization, the merchant will post the transaction. When the transactions "posts," the "pre-authorization" drops off. The posted dollar amount is the final amount of the purchase. Keep in mind that gas stations may pre-authorize a specific amount to your Fancard. To avoid this, we recommend going inside and prepaying for a specific amount of gas that you want.
The reason why the "pre-authorization" and "post" amounts can be different is best illustrated with the example of leaving a tip at a restaurant. When a restaurant requests a "pre-authorization" on your Fancard, 20% is added to this amount to estimate the tip that you will leave. Once the transaction posts 24 hours later, the amount will reflect the actual tip amount that you left.
Essentially, a "pre-authorization" is the "hold" that is put on your Fancard account at the time of making a purchase. The merchant then has 24-48 hours to actually post the final dollar amount and collect the funds from your account.