On his 19th birthday, the MC has completed his 18 years of life. Therefore, he will not receive his inheritance (Lynette's check) until his 19th birthday. The inheritance (Lynette's check) came through a letter from an attorney.
Lynette's personal items were probably given away by someone who is only now learning that the MC is family. A grieving woman is present at the preparation party. She may also have received a letter or diary from Lynette from someone recently deceased stating that the MC is family. This woman immediately reads the last pages of Lynette's diary and learns that the MC is her child. That's why the MC got his mother's diary so late.