where do I enter postage for my home business expense

You will need to add expenses.  For postage, you can choose "other miscellaneous expenses" and add the description "Postage."

Here is how to do this in TurboTax Online:

• Log back in, navigate to Federal > Wages & Income > Self-employment income and expenses > Add/Edit
• Find your business and choose "Edit"
• Now, adjust your income or expenses as needed

If you are using Desktop:

• Navigate to Federal Taxes > Wages & Income > Business Items > Businesses Income and Expenses (Sch C) > Update
• Find your business and choose "Edit"
• Now, adjust your income or expenses as needed