Sefer Divrei Tzadikim explains Yitzchok’s love for Esav by saying that his intent was to arouse Divine compassion for Klal Yisroel. He wanted Hashem to see that he loved his son, despite his evil ways, merely because he did one good thing of providing him with food. He did this so that He would show love and compassion for the Jewish people even when they act improperly, as they are full of many mitzvos.
We see from this that Yitzchok was aware that Esav was a bad person, but he was going to give him the brachos as reward for the one mitzvah that he did. It is thus certain that a Jewish person who does any mitzvos will receive untold reward.
In this vein, Sefer Tzipurei Tzadikim Hachadash (Ois 20) relates that Rav Aharon of Karlin zy”a was once learning Gemara when he had a kasha for which he could not come up with an answer. This caused him a lot of heartache, and he davened to Hashem to send him the answer. Suddenly, a wagon driver who had recently died entered his room. He said that he had been sent from the World of Truth to give him a message. He then handed him a note. The Rebbe looked at it and saw that the answer to his question was written there.
The Rebbe asked him how things were going for him in the Upper World, and the wagon driver answered, “Things are very good for me there. They gave me a big, beautiful room, where I sit alone. When they need to send a messenger to this world, they send me. Only one thing is not so good. I am alone in the room with no company.”
The Rebbe asked him, “Why did you get such a great reward? I knew you as a simple man!”
The wagon driver said, “It’s true that I was a simple person, but I had one big zechus. I once was driving my wagon through a forest, when I saw a Jew hanging from a tree. He was still alive, and he yelled to me to help him. I hurried off my wagon and cut him down, thereby saving his life. As a reward for this, I was given my own room in Gan Eden.”
The Rebbe later said that this story helped him understand the Gemara ( Avodah Zara 10B ) that says: “There are some people who acquire their world (olamo) in one hour.” Why does it say that they acquire “their world”? Why doesn’t it say that they acquire “Olam Haba”? The answer is that even simple people like this wagon driver who don’t really deserve Olam Haba can acquire their own portion in the Upper World as the result of one good deed.