Oh... I think only you can help here. :)
I don't know ...

When buying a project, you get access to all the files in it. You don't buy individual files. You can pay more to access certain files that have a special price, if there are any, but you still pay for the project. As a whole. Your library contains projects. Not files. You can't add files to your library.

When the bundle expires after 1 hours, will I lost them again, and have to buy it?

Yes, have to buy again. I tried it.