To create a SQLite database with the specified tables and requirements using SQLAlchemy, you can use the following Python code:

```python
from sqlalchemy import create_engine, Column, Integer, String, Numeric, DateTime, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

Base = declarative_base()

class Customer(Base):
    __tablename__ = 'customers'
    
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String, nullable=True)
    balance = Column(Numeric(precision=10, scale=2), nullable=True)
    credit_limit = Column(Numeric(precision=10, scale=2), nullable=True)

class Order(Base):
    __tablename__ = 'orders'
    
    id = Column(Integer, primary_key=True, autoincrement=True)
    customer_id = Column(Integer, ForeignKey('customers.id'))
    date_placed = Column(DateTime, nullable=True)
    date_shipped = Column(DateTime, nullable=True)
    notes = Column(String, nullable=True)
    amount_total = Column(Numeric(precision=10, scale=2), nullable=True)
    
    customer = relationship("Customer", backref="orders")

class Product(Base):
    __tablename__ = 'products'
    
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(String, nullable=True)
    unit_price = Column(Numeric(precision=10, scale=2), nullable=True)

class Item(Base):
    __tablename__ = 'items'
    
    id = Column(Integer, primary_key=True, autoincrement=True)
    order_id = Column(Integer, ForeignKey('orders.id'))
    product_id = Column(Integer, ForeignKey('products.id'))
    quantity = Column(Integer, nullable=True)
    amount = Column(Numeric(precision=10, scale=2), nullable=True)
    unit_price = Column(Numeric(precision=10, scale=2), nullable=True)
    
    order = relationship("Order", backref="items")
    product = relationship("Product", backref="items")

# Create the SQLite database and the tables
engine = create_engine('sqlite:///system/genai/temp/create_db_models.sqlite')
Base.metadata.create_all(engine)

# Insert some sample data
from sqlalchemy.orm import sessionmaker
from sqlalchemy import inspect

Session = sessionmaker(bind=engine)
session = Session()

customer1 = Customer(name='Alice', balance=1000, credit_limit=1500)
customer2 = Customer(name='Bob', balance=2000, credit_limit=2500)

product1 = Product(name='Widget A', unit_price=10.50)
product2 = Product(name='Widget B', unit_price=15.75)

session.add(customer1)
session.add(customer2)
session.add(product1)
session.add(product2)

session.commit()
```

This code snippet sets up the database schema with the specified tables and relationships using SQLAlchemy. It creates the SQLite database at the specified location and inserts sample data for customers and products. You can extend this code to add more data or make further modifications as needed.