Since using the iTunes Store is tightly integrated with your iTunes account and your profile, the iTunes Store knows who you are most of the time. This means that it has all your credit card information available, which makes "one click purchases" possible. Especially since the iTunes Store no longer offers a shopping cart purchasing option (more on that below), anything you want to purchase is literally just a click away. This tutorial explain how you can purchase from the iTunes Store, and the various bundling options you have. We will also show you how to find free content from the iTunes Store!
Purchasing Options from the iTunes Store
Any item for sale in the iTunes Store presents itself more or less the same way: you can purchase entire albums (music) or seasons (TV shows), but also a single song, or a single TV show. Since the iTunes shopping cart no longer exists, any purchase you make is held under your iTunes account, and charged to your credit card once you reach about USD $40, or after a certain period of time has elapsed. Since the iTunes Store is a venture of Apple Inc., you will have to pay taxes if you live in the United States (because the company Apple has a legal representation (store or otherwise) in all 50 states). 
When you click Buy Album, or the Buy button next to a particular song, iTunes will initially get your confirmation every time you are about to make a purchase. After a while, once you get accustomed to the iTunes Store purchasing experience, you will probably want to tick the Do not ask me again checkbox, and then truly purchase content with a single click.
Pre-purchasing checks by the iTunes Store
In the next step, the iTunes Store will check several things on the item you are about to purchase: first, it will check that you are in fact allowed to purchase this item is this particular country store: you can browse other countries' iTunes Stores, but you are not allowed to make purchases from any country store other than the country in which your credit card is registered. In other case, a song has been pulled from the iTunes store of your country between the moment where you look at the album and the moment you decide to purchase the album or a particular song (this happened to us - very weird). In that case, the iTunes Store will give you a message like "This content is not available in your country" (from memory).
The second check the iTunes Store runs before charging your account and downloading the album/song/TV show, is to make sure that you have not already purchased this particular item. If you have, iTunes will display the following message: ''You have already purchased this item. Would you like to buy it again?".
iTunes is giving you a last change to change your mind, but still lets you purchase an item you already bought in the past: this is useful if you bought an item from your iPhone or iPod touch, and lost the iPhone or iPod before you got a chance to synchronize it with your Mac or PC.
Where is my iTunes Store Shopping Cart gone?
Since iTunes 9, the shopping cart is no longer an option in the iTunes Store - instead, Apple introduced the "Wish List". Fortunately, if you were using the iTunes Store cart, the content you had in there is not lost: when you upgrade to iTunes 9, the content of your cart was automatically moved to, and can be accessed through your Wish List.
A later iTunes Store tutorial will be solely dedicated to the Wish List feature, and how to make the most of it. If you are familiar with the Wish List feature on Amazon, the iTunes Store Wish List will be very easy to learn!
The implication of this of course, is that you can now only purchase content as-you-go. If you do not plan on using the Wish List, however, you can use it just like you used the iTunes Store cart in the past. Our guess is that you will quickly forget the cart, as we have, and enjoy the advantages of the Wish List. But since you add more readily items to a Wish List than to a cart, the dollar amount of content put aside quickly becomes astronomical!
Complete My Album
The iTunes Store also includes a feature called "Complete My Album". Most albums offer you a special price, lower than if you were to purchase songs individually. The exception lies with long pieces of music -more than 10 minutes long- which typically cannot be purchased as individual songs: such items usually display an "Album Only" notice - in that case, the only way to buy the song is to purchase the entire album.
This exception aside, clicking the Buy Album button for an album from which you already purchased content (from the iTunes Store that is) will give you a confirmation message and allow you to purchase the remainder of the album at a discounted price - the album price minus $0.99 (or the price of a song in your country store and currency) for each song you have already purchased from the album.
Here is an example of Complete My Album: notice how the purchased items are grayed-out (disabled) - you either have the option of purchasing individually the remaining tracks, or to complete your album at a reduced price (notice the Buy Album button on the left, customized to reflect your purchasing history).
Just click on the revised Buy Album button, and the songs you have not yet purchased will be downloaded (and your album, completed!) The new songs will automatically appear as part of the original album you purchased (if you sort your music by album / singer - if you sort by date purchased, these last songs will be displayed at the very bottom of the list).
iTunes Store tip: Note that this feature of the iTunes Store only works on an album-level: in other words, you will not get this offer if you purchased a song from a compilation and later on decide to buy an album containing that song. But you would get the Complete My Album offer if you purchased a few songs from that compilation, and then decided to purchase the entire compilation. This offer obviously only applies to songs you have purchased from the iTunes Store (not third-party sites).
Avoiding duplicate purchases from the iTunes Store
As we briefly mentioned earlier, the iTunes Store will prevent accidental purchases. Although not historically true, the iTunes Store now gives you a warning before you are about to purchase an item already purchased under your account profile (your Apple ID). The iTunes Store does not prevent you from re-purchasing content (in case you accidentally deleted it, for example).