What should I do if the courier company signs for the goods privately?

The easiest way is to tell the mailroom downstairs not to accept your express mail any more, or you will find the mailroom responsible for the goods. Secondly, if the courier company signs the name by itself, it is against the rules, but if the doorman in your mailroom is alive and stamps the mailroom, they are not responsible. You can't clean them. You can only be smart and find a way to deal with them yourself. If you have to, the next time they send the goods to the mailroom without informing you, you can say that you have not received the goods, and the mailroom can also say that there is no one. If they sign for it themselves, the staff in the mailroom will not admit to receiving the express delivery. You torment them once or twice, and he will definitely call you next time.