I did it. It took too many hours of fighting, thanks to bad and conflicting documentation (mostly on the API side) and a good measure of clueless newbieness, but I did it.
But oh man, that debugging. Every single attempt, I had to redo my log in on the grocery store’s website (since I couldn’t yet get the authorization token that would have let me skip it). If I ever log in manually to the grocery store’s website again manually, it’ll be too soon.
Next goal: Put something in my shopping cart!