Olivia - there are two different ticket options. One includes just the house (offered every day), and the other (only offered for certain days on the website) includes the 30 minute introduction and the house.
The ticketing rules changed recently, and the website, finicky before, is now even more trouble. I was looking with a coworker for her trip. We looked every day, and it went right from not being available (long after it her dates were supposed to be available) to sold out.
I hope you have better results than she did.
BTW, my brother found out that the website processes the transaction through France (or at least, it did so last year). He had gotten his card approved for transactions in the Netherlands and Belgium, since he was going to those two countries, but his Anne Frank House ticket purchase was still rejected - he had to get France added to that list!