Welcome to WebmasterWorld Mikala!
First - a thread you should book mark for future reference: [webmasterworld.com...]
Second - Online services like MonsterCommerce or Mountain Media have the benefit of letting you get your feet wet without the up front expense of building something.
The added advantage is that you're going to save some additional money (and the pain from smacking yourself in the head) when you realize that the functionality of the custom cart you bought isn't quite what you wanted.
Better to start out easy - especially if you don't have a webjockey on staff to help guide you - and take notes as you go!
That being said, there will come a day when you realize that you're limited by these online services. They may work well for you for a few years or more or just a few weeks. It really depends upon how much you learn and how quickly.
The next logical progression will be to move to an off-the-shelf package and have it modified to your liking. These come in a variety of different flavors. Between now and then, research and read and keep notes on what you do and don't like about the service you go with!