Is it permissible for an employee to take the special offers (e.g., ball or cup) that come with items he purchases for the company?

You are not permitted to take the offers for yourself, because they are secondary to the primary item, which is the goods you purchased. The primary item belongs to the company, so the secondary item also belongs to it. If they were given to you on the basis that they are for the company, then give them to the company's owners; otherwise, do not take them.