Hello,

As far as I can tell, AP invoice saved in the system cannot be deleted, even if it is in Preliminary status.

All you can do is to cancel it (using right-click menu option ). 

In case of AR, it might depend on the invoice source. I think Project invoice and Instant invoice could be deleted using standard Delete operation while in Preliminary status. I am not sure if this is applicable to Customer Order invoice as well. AR invoice in status other than Preliminary can be only credited or corrected.