I want to file this return myself, with a partial payment. Why can't I do that like I've always done??

In the FILE section, you should be asked how you want to pay your Balance Due.  

 

Select 'I will mail a check' if you want to make a partial payment. If you choose to pay with your bank account or credit card, TurboTax will charge the total tax due.

 

Here's more info on How to Pay Tax Balance Due.

 

@mmcintosh1600