Title is the question. When should a relationship in an ER diagram be given its own table in a RDBMS? For instance, one mail courier(with attributes eid and surname) can deliver a number of packages but a package(attributes,pid, sent_By, going_to) can only have one mail courier. Would it make sense to make a table for the relationship called delivers(with an attribute of the time that the package was delivered)? or should the eid of the mail_courier and time_delivered from the deliver relationship be added to the package entity? Also, what would be an example when you would not want to add the attributes to the package entity?
I think what you are trying is to create a one-to-many relationship between two entities. And for that, there is no need to create a separate table; as you mentioned in your question, just add those two attributes to the package table.
Where you would need to create a separate table is when you want to achieve many-to-many relationship between two entities. For example, take twitter's followers. One user can have many followers and a follower can follow many users. You can't do that the relational way without creating a new table with just those two columns.