no, I don't think Aziraphale was under any influence from the coffee or something when he made that choice in the last scene
given what we know about Aziraphale, his difficulty in grasping his own morality and his strong belief that people (and even heaven) can change it makes perfect sense that he would take the chance to finally make a difference
but the thing is, he really thought he would be able to do it with Crowley by his side, he wasn't expecting him to deny the offer because he still thinks heaven is inherently good and therefore it would be an obvious choice
but it isn't, not for Crowley who was kicked out of heaven for asking questions, who's seen heaven's heartless attempt to kill the most important "person" in his life, who's seen what heaven is capable of doing for what they believe to be right, not good, just right (for them)
they could've still be them but Aziraphale had to make a decision for what he thought was right and that meant loosing the chance of being with someone he loves
it wasn't out of character and I really do believe that the internal struggles faced by the angel will play a big part in the third season as he discovers more about himself, heaven, hell, good, bad, humanity and love