You are absolutely correct. There is no Other Common Business Expenses line on Schedule C.
That is a section in the EasyStep interview for business expenses in TurboTax Self Employed. I suspect you are working in Forms mode. If your switch to EasyStep, you fill find the Other Common Business Expenses section under the Business tab.
If you choose to remain in Forms mode, you may enter expenses such as PO box rent on Schedule C, Part V, Other Expenses. Enter a brief description and amount for each expense.