Had to laugh at the customers comment about how the cake slices were not a real size slice...tell her that she could Super-size her cakes, but there will be an extra fee to do so. Seriously though she's not worth a massive migraine headache - I wouldn't call her back...let her make her own wedding cake, we all know how easy it will be 
WHEN she calls you back begging for forgiveness, the reality is going to be that you're already booked to organize belly button lint 
You truly don't need to have someone grouching at your backstage crew like that for ANY reason, especially on an order that you'd be getting paid squat for..
I agree with the others, this one is something bad waiting to happen. She told you just to tear up the order, fine, consider it torn and trashed... Which means, even if for some reason you do feel compelled to do the cake, your quote has gone up to living wages AND you have the clause in your contract about no one else's cake but yours.. It also sounds like someone who would have complained about how the cake was done expecting a $5000+ cake for $150...
Good luck!
She sounds like a raving nutter
ROTFLMAO!!!! That sounds too funny! 
Oh and as for your unhappy customer?
NO CAKE FOR YOU!
I would not call her back, and when she calls you back, tell her you are booked. $1 a serving is ridiculous, that went away in the 70's. And to complain about the size of servings......... you are good to be rid of her. Don't let her convince you to do the cake at all.
Well............... maybe for $5 a serving........... LOL
Yep. When she calls back to say she couldn't find ANYONE ELSE ON THE PLANET to sell her a wedding cake for $1/sv...
Just be professional and polite, and cheerfully say "Oh, yes, I do remember that you had called to let us know you weren't booking this date (NOT 'cancel my order'--she never DID order), so it has already been booked. Thanks for calling, though! BUH-Bye"
Don't give it another thought--a nicer and better bride will come along anyway.
Quote by @%username% on %date%
%body%