I'm using the PC version not online, though I see in the chart you linked it does show that Schedule C is included in Deluxe as well as Premium, so at least for online it suggests you don't need Premium.
Either way I looked back at my return last year and I did enter items for Schedule C box 27a as well as listing those items in Part V of Schedule C, and I did this with the same software package - TurboTax Deluxe for PC.
Did TurboTax reduce the functionality of the Deluxe software just for the PC version then? Or am I missing some way to add to box 27a? I'm able to do the rest of Schedule C through my current version.
Yes you can enter those. Line 27a comes from Part V Other Expenses line 48. It is under Misc expenses.
Under Business Expenses, Click Start or Update by Other common business expenses
You should see the list of expenses and amounts you entered.
Then click the start or update button by the expense you want to add or edit.
Other Miscellaneous Expenses is the 16th item under Expenses - Click the Start or Update button