Question about being paid per day

Hi I just finished dog sitting for the first time. I picked the dog up on the evening of Dec. 20 and returned him this morning.

I charged $40/day (undercharging I now know) but the owner did not pay me for Dec. 20 or today. I understand they weren’t full days - and that we both should have communicated better. But how can I ask for the $80, I feel as though I deserve? (Or at least $40 because it was two half days).

Or should I just leave it be?

This arrangement wasn’t made on any kind of business site so I don’t really have back up