Shop or Me?

[Sparki 1]

I didn't notice that... Makes sense 

looks like nasty sticky grease preventing those pawls from rising. 

[Paulst12]

Those paws look full of grease which usually makes them sticky and hence why they don't engage. The hub should be lightly oiled so they lift easily and fully engage. Basically I think the shop just removed the previous axle and put a new one in without checking all the working parts. Their fault for not doing that, however your fault for not checking it once it started slipping on your 30km training ride. To sort that out takes 10 minutes max.

[PhilipV]

Sounds like you saved yourself a lot of drivetrain wear by not finishing the ride.