How to add payment method field to the footer of an order summary view?
Why is it? Fields are meant to go in the fields section and not in the footer. 

An easy workaround solution is to create an attachment view contenting only the field in question and attach it after your current view. 


To do this, in your view, click on +Add and select attachment.


 


Then you should have a cloned copy of your original view. When you add or remove fields make sure you select at top This attachment (override). If you do not do this, your changes will affect all of your views. 





In your first/original view: In footer click on Add and select Global: View area. Select your attachment view and check mark the inherit contextual filters box. 





  Note: If your first/original view is a Master, when you add the attachment, you will not have the For:
  Display box,  such as in the last picture (on step 3). 
  
  If that is the case, all your views will get an attachment, including the 
  attachment. Therefore, you will have to switch to the attachment view and 
  select "This attachment (override)" and remove the footer from the attachment.