Here's an example SQLAlchemy code to create a SQLite database with the specified tables and fields:

```python
from sqlalchemy import create_engine, Column, Integer, String, Numeric, 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'))
    amount_total = Column(Numeric(precision=10, scale=2), nullable=True)
    notes = Column(String, nullable=True)

    customer = relationship("Customer", back_populates="orders")

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)

    order = relationship("Order", back_populates="items")

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)

engine = create_engine('sqlite:///system/genai/temp/create_db_models.sqlite')
Base.metadata.create_all(engine)
```

To add sample data to the tables (customers and products), you can create instances of the respective classes and add them to the database session, and then commit the changes. Here's an example:

```python
from sqlalchemy.orm import sessionmaker

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

# Add sample customers
customer1 = Customer(name="John Doe", balance=1000.00, credit_limit=2000.00)
customer2 = Customer(name="Jane Smith", balance=1500.00, credit_limit=3000.00)

# Add sample products
product1 = Product(name="Product A", unit_price=10.00)
product2 = Product(name="Product B", unit_price=20.00)

session.add_all([customer1, customer2, product1, product2])
session.commit()
```

This creates a SQLite database with the specified tables and fields, and adds sample customer and product data. Feel free to modify the code to fit your specific requirements or add more data as needed.