Reconsidering Star Wars IV in the light of I-III.
Fortunately, before coming to Tattoine, R2 had already arranged transport, which is waiting at Mos Eisley, under the command of the Rebellion's other chief field agent and espionage asset. Chewbacca.
20 years earlier, Chewbacca was second in command of the defence of his planet. He's there in the tactical conferences and there on the front lines and is a personal friend of Yoda's. When he needed reliable people to join the embryonic Alliance, who else would Yoda turn to but his old friend from Kashykk? Given his background, there is no way that Chewie would spend the crucial years of the rebellion as the second-in-command to (sorry Han) a low-level smuggler. Unless it's his cover. In fact, Chewie is a top-line spy and flies what is in many ways the Rebellion's best ship.