What is the best way Reinforcement learning, RNN or others to predict the best action we have to take to maximize sales?
I don't know if reinforcement learning is right for this problem.  A reinforcement learning model needs a simulated "game" environment where it can interact and select actions initially at random.  But real customer reactions are hard to simulate, and you don't want the model randomly sending out spam e-mails before it's trained.

There are other ways to train a model to recommend an action based on existing data.  If you want to keep it simple, you can perform a regression to estimate each action's impact on likelihood of a sale.  If you have more data and want to get more complex, perhaps look into recommender systems (Netflix, Amazon, etc.)