Step-by-step explanation:
In Python, you can create a constant by convention (since Python doesn't have built-in support for constants) by using uppercase letters for the variable name to indicate that it should not be changed. Here's how you can declare a name constant for a commission rate of 0.03:
```python
COMMISSION_RATE = 0.03
```
By using uppercase letters for the variable name, you signal to other developers that this value should be treated as a constant and not modified. However, please note that Python doesn't enforce true constant behavior, so you should follow the convention of not modifying this variable in your code.