Is there a way to check if the books I returned made it back to the library?
Yes, you can check this in your Account in the Book Catalog. Login and see if there are any items listed under Checkouts. If the list is empty, you can be sure that the books have come back and been checked in: taking the book off your account.
On the other hand, if you find that list in your account, even after a week of shipping the package, contact the Online Librarians immediately, so that we can track the package.