Attack of Math part duex

So I've got this other post I'm working on but this is more important right now.
Today I went and paid a bill and then headed over to a local Blockbuster that is on the way home.
I stopped in and they have a promotion listed right in front of their used Blue Ray movies that says 5 for $20 and does not list a price point.  All, and I mean ALL of the Blue Rays in front of the sign are $9.99 each.

So, I grabbed 5 that I wanted and headed to the register.  I had to wait a minute as the obviously new cashier, CSR, GSR, WTFever she is was ringing up someone else and while waiting I grabbed an over priced soda because it's a long drive home and I need more sugar in my fat stream.

Finally it's my turn and the new chick is on the phone with someone that I'll be kind and assume was her manager who was not in the store and getting advice on how to be less stupid.  While gossiping on the phone she rings me up and totals me out and I'm about to swipe my card when I notice the price is over $10 too high.  She hangs up and I tell her the price is wrong.
She looks at everything and tells me that it's right and that my discount came off.
I ask if the Blue Rays are 5 for $20 and she says "Yes, they are" and I tell her that I only have 5 Blue Rays that I handed to her.
She tells me that the difference is coming from my Soda.
I told her the movies were 5 for $20, the Soda is not costing me over $10 since my total is $34.xx.
I ask her if she maybe scanned one of the movies twice by accident because a $9.99 movie is $10 bucks and my price is wrong by $10 bucks and she tells me that she did not make any mistakes at all.
I ask her to review her information because I'm not paying $34 for $20 worth of movies and a soda.
So, this is where she has computer problems and has to call her manager no less than 3 times and be on the phone for 20 minutes or so.
A line is forming.
Finally accidentally does the right thing and the computer is working again and she goes through her screen and tells me everything is fine, Everything is only on there once and the discount was taken off.
I ask her how much the discount is and she tells me $19.99.
I have to show her how to do the math and point out that 5 $10 movies equals $50, subtract $20 from that and you are left with $30 which is NOT what the promotion is listed at and NOT what she told me the promotion was.
I wish I could fucking make up the next sentence out of her mouth.
"Well, the difference is going to be your tax."
So I ask her what the tax rate is in Blockbuster because Las Vegas is only a little over 8% which is NOT going to come out to be $10 worth of tax.
I ask her if the promotion is wrong and the Blue Rays are 4 for $20 instead of 5 and she tells me that no, the promotion is 5 for $20 but she doesn't know why the computer is not giving the correct price.
She then tells me that she is going to make a horrible manager since she can't even figure out the computers and cannot come up with the correct price for my purchase.
So, I got the manager in training (which I'm hoping was just the shift supervisor and NOT a Store Manager in training) who is supposed to be one of the smart ones and she can't figure out that there is no way that there is $10 in tax.
She also tried to tell me that maybe my soda was more expensive than we thought it was.
More expensive than what? 2 bucks?  Are you telling me that my soda really cost $10 and this is the fucking reason you never sell them?

I wish my life were a movie so I could just go out to my car, pull out a bazooka and blow the fucking place up and then drive away.