I'd recommend getting another brake to go with the coaster. I ran a similar set up (admittedly with a kid on the back too), and I had a little trouble stopping on downhills when fully laden. The brake would just creak a lot and sound in pain, or just lock out the back wheel, both not fun with a heavy bike on a steep descent. Looks like a fun shopping ride.
I'd recommend getting another brake to go with the coaster. I ran a similar set up (admittedly with a kid on the back too), and I had a little trouble stopping on downhills when fully laden. The brake would just creak a lot and sound in pain, or just lock out the back wheel, both not fun with a heavy bike on a steep descent. Looks like a fun shopping ride.