I couldn't say for certain which bundle builder this website uses. It's very possible that they have used a developer to create a custom-coded solution for their needs. With that said, I've compiled a couple bundle builder apps that work in a similar fashion and will accomplish what you're looking for:
They both offer an interactive module where the customer when add in the various products/variants to build their bundle. Seeing the cost of the bundle update in real-time. I hope that helps! If there is anything else I can help you with, please let me know.