I ordered food supplements which have no "Kosher for Passover" indication. The shipment was delayed, and I was now informed that it will arrive during Chol HaMoed. Can I include the items that have not yet been delivered to my house in the "mechirat chametz" that I will do before Pesach?

There is a difference of opinion among halachic authorities whether a package which is in the hands of the postal service, is already considered yours or not. Accordingly, if you also sell the chametz to a gentile (so it's definitely not yours) and also "mafkir" and disown it so that it's not yours, so either way you are covered. This definitely is not something that you should plan on doing, but being that it's b'dieved (unplanned and not your fault), and also there probably is no real and genuine edible chametz there, you can rely on the 2 aforementioned solutions. Obviously, when it arrives, place it immediately with the chametz which you sold, and don't use it until after Pesach.