If a Jew brings in mail for me (From right outside the house, In a place with an eruv) That contains muktza on shabbos can I benefit from the item? I did not ask them to bring it in. They just have the practice of bringing in the mail on shabbos and sometimes my mail is mixed In with theirs. I do not know if they know that they should not be bringing in the mail on shabbos or not.
You can benefit from it after shabbos without a problem. The item is muktza so you are not going to benefit from it on shabbos itself. Regarding benefitting from tit after shabbos, since he only did an issur d’rabonon, you may benefit from it after shabbos.