Customer Level is fairly easy to increase. All you need to do is purchase and sell items to the Rosen Queen Co. Store. It is best to complete this by Episode 10 as you will need the best equipment to complete them.
Stage 9-4 is a good Stage to Power Level in the Late game. Mt. Ordeal Stage 4 is also very good if you are post game. These stages will earn you a lot of HL when you complete them. Which you can use to buy expensive equipment.
The Max Level is 12. The Customer Rank will increase as you Purchase/Sell equipment and Items however you will need to use the Cam-pain board to upgrade your Product rank so you have access to the higher end Equipment. You also cannot upgrade your Product rank past the Customer Rank.