Almost always “Thanks, [T’s first name].” She typically just emails to send an invoice for sessions; I submit the claim to insurance instead of her.

On a side note, I taught college classes for several years. I used to sign my emails “Regards, firstname” because I thought “Regards” seemed warm. Then a student mentioned one day that my “cold” email closing seemed incongruous with my warm personality in person. I guess perceptions differ on that one.