I "was"guilty of pricing too cheap, I seemed to have a hang up when I measured up (Carpets) and the price went over the £100 mark.... The customer could clearly see that I was lacking in confidence and would knock my prices down.... It's taken a while but now I tell them what the price is with more authority.
I did quote for a regular who always knocks my price down, so I added £40 on top of my genuine quote.... they felt so good when they bartered me down by £30

You're not just a pressure washing operative, you're a manager, stock manager etc etc.... Make sure you're covering your costs..... All the best!