The "detectives" seem convinced that a PayPal transaction that occurred after the death of one of victims is suspicious. They bring that up and don't check that out right away, at the way this episode is edited. That should have been their next step. A transaction appearing on a statement after someones death is easily explained. An online pre-order could easily explain a charge that would appear after someones death. Also standing orders could explain it. You can have a monthly purchase set-up on any one of several websites eg. Amazon.
But investigating that right away or placing it at the beginning of this episode would quickly put a damper on the tension they're trying to establish in this episode.