Is there a store that we can just walk into and buy some things and walk out? Or do we have to pay to enter?

There's a gift shop. You don't need to pay admission to go into it.

The gift shop is free to visit. There are a lot of great items to buy and friendly staff to help.

They have a gift shop as you enter, you can also purchase tickets for the tour there and there is no fee to enter the building.