I'm going to yell at a customer
Was the customer rude to you or something?
No, but I was working all day long with no breaks
I know being tired and such doesn't make you feel much like being polite to yet another customer. However, you shouldn't take it out on a random person because your day was difficult.
That's the way it is, the customer always right. But sometimes I just want to yell at them.
I can understand the way that you feel, but remember that the next customer who shows up isn't to blame for the stress that has been building up on you.  This person just needs to go about their day, the same way that you do every time you need to go to a crowded post office or busy restaurant.  Just find a way to relax after work.