For one, when sending cards to multiple addressees (holiday cards or event invitations, for example), Apple treats each card as an individual purchase from the Apple Online Store, rather than bundling purchases like on the iTunes Store. This means that for each card sent, Apple sends two emails (one for the order and another when it ships), sends a push notification (upon delivery), and charges a credit card.
When purchasing multiple apps or songs from the iTunes or App Stores, Apple aggregates multiple orders into one credit card charge. This lowers the company's credit card fees and, as seen above, avoids clogging bank statements with dozens of charges. It also avoids having a credit card locked due to the bank assuming that 8 charges for $3.08 in 10 minutes is fraudulent activity.
The most annoying of the app's quirks is the tendency of the app to fail during checkout, occasionally requiring multiple attempts to purchase a card for no discernible reason. When the purchase fails Apple still authorizes the credit card, resulting in even more charges appearing. This weekend, when ordering 37 cards, my credit card was charged for 52 transactions, though the additional charges eventually disappeared.
The Cards app is a convenient way to send personalized cards through the mail, but it needs a few tweaks, including a native iPad version and the ability for users to easily send cards to multiple addresses without dozens of individual credit card charges.