> Deployments are not zero-downtime at this point and I’m planning to switch to an AB-style deployment model once the site has enough traffic to warrant it
Do you mean blue-green deployment ( https://docs.cloudfoundry.org/devguide/deploy-apps/blue-gree... )? In any case note that you can do that even with just a single compute–just run both instances of the app in the same compute, on different ports–say, 3000 and 3001–and use a reverse proxy like Nginx to load-balance incoming HTTP traffic between them. Then you get blue-green deployment almost for free.
It's similar to rails and Django model but in racket (a dialect of lisp based on scheme branch of lisp family).
This is a very good example for learning racket lang if the sourcecode is open for some parts.
I am sure some member of racket community will bring up library specially designed to address e-commerce as it is in Ruby and Python eco-system.
Hopefully it can be like what Paul, the founder of YC said in his blog post "beating the averages. 
"In Lisp, there are programs called macros. They are programs that write programs."
Interested to know if typed racket was used for this, and if so, what the experience was like!
I'm a fairly experienced Racket developer (6 years) but it was my first time trying Typed Racket. I ended up not keeping the part of my code that I typed.