What is a Junker?

Everything changed; planes flew, and a businessman tossed a sack of letters out of a Junker and invented airmail. - (Garcia Marquez: Memories of My Melancholy Whores)

The closest match I found on Wikipedia's disambiguation page for Junker was Junkers which might refer to German aircraft designer Hugo Junkers. But, the text says "Junker" not Junkers.

What does it mean in this context?

The passage certainly refers to the German aircraft company Junkers: the ‘s’ has been omitted. The bit about flinging deliveries out of open cockpits (including the first aerial bombs, which were simply grenades) comes from the very early days of aviation, and seems not to be specific to airmail as far as I can tell.

After the Great War, under the Treaty of Versailles, Germany was for a while prohibited from building aircraft, and generally from creating any arsenal of war equipment. Of course it did so anyway. Some of the medium-sized aircraft from that period were essentially designed as bombers and saw service as such during WWII, but were originally categorised as mailplanes. The most famous such example is the Dornier 17.

Less prominent was the supply to the USA of several mailplanes by Junkers, as outlined here. The most influential Junkers model to see WWII service was the versatile and feared Ju88 medium bomber, which essentially superseded the Do17 and excelled in other roles as well.

Junkers also produced the Ju87 ‘Stuka’ dive-bomber, which became symbolic of the trademark German technique of Blitzkrieg: precision bombing of enemy positions and rapid advance by armour, with infantry to take swift control of the shocked defenders. (The Ju88 and other models also undertook dive-bombing, but without the Stuka’s characteristic screech.)

All of this might be why Marquez uses ‘Junkers’ as a particularly evocative aircraft manufacturer, a name that needs no explanation.

The spelling error remains odd, though. I initially speculated that the fault in the quoted passage might have been in the translation from Spanish: just possibly the American translator (in 2005) Edith Grossman might not have had the knowledge of the inter-war Luftwaffe that an English boy-child of the 1970s (i.e. me) had had. Perhaps she had assumed the ‘s’ to represent an erroneous plural, and had mistakenly ‘corrected’ the grammar.

Not so, as far as I can tell. This file appears to give the original Spanish text, as ‘un hombre de empresa tiró un saco de cartas desde un Junker e inventó el correo aéreo.’

Perhaps Junkers is known as Junker in Colombia...?

Even so, we are left with the mystery of this apparently being written by Marquez in the first place, and then getting past both his editor and then his translator as well. You seem to have uncovered something potentially revealing about Marquez’s publishing history. At the very least, why did Grossman not correct the (well-known) manufacturer’s name?

Since the word "airmail" is mentioned I would indeed think that he meant the German plane and misspelled it. See this Wikipedia article about the plane https://en.wikipedia.org/wiki/Junkers_Ju_88