Historical In-Joke - The punchline of the "Uniforms" comic. Here, have a ruined joke: Hugo Boss already made Nazi uniforms, so he's buying what he has already. Not just that, but Hugo Boss made SA uniforms, SA being the purged branch during the Night of the Long Knives. He's repeating history.
Hitler's testicles... er, testicle... uh... penis.
The 20 July plot, which is averted because Hitler thinks the bomber is bourgeois.
Incredibly Lame Pun: Pretty much all of Hitler's shirts feature some Nazi-related pun, including "Three Reichs and You're Out", "Heilvetica", and "Blitz or GTFO".
This is because it turns out that Hipster Hitler's arch-nemesis is... BROSEPH STALIN!!
Merchandise-Driven: With only two strips up, the creators were running a poll as to which t-shirts based on the strip readers would most like to buy. By strip three, an on-line store was up with a dozen different t-shirts for sale. The entire strip seems to be little more than a cynical ploy to create a link-bait property as a vehicle to sell things to exactly the kind of people the strip parodies (not talking about Nazis here). However, we can all forgiven this because the strip really is just that funny.