What is a typical charge for a cake tasting?   She said she doesn't need much, but I still have to make more than one or two cupcakes.  

Personally I I think that there should be a minimum order.

I am already making a two tiered fondant cake with embellishments & topper for $100.  oh, and a smash cake.

I already know that I under priced myself.  At that time I had no idea that she would want a cake tasting.  Is it typical for people to do cake tasting for birthday cake?  

No, it isn't. I'd definitely charge her for a dozen cupcakes if she wants a tasting, since you can't make one cupcake for her to taste. You've WAY underpriced yourself for what she's buying already. Any more time and materials spent on that order for free will mean that you're probably working for less than minimum wage, don't do that to yourself.

Personally, I'd tell he that you don't do tastings for birthday cakes, but if she wants to buy something else in the flavor she's thinking of for full retail then you can do that. Do NOT let yourself be bullied into doing free or cheap samples just because she's already placed an order.

She didn't want just one but two different flavors. I told her I can't just make her one or two cupcakes.  Needless to say, she is going somewhere else.  I think she was just expecting a lot and I personally didn't want to deal with it anyway. I just got that gut feeling.  I feel relieved.

I am not  going to make a 2 whole batches of cupcakes, just so she can have one.  I'm not going to spend my money and work for free.

Good, trust your psycho radar. If it feels wrong you don't want to get involved with it.

Here's a good idea.  Create a policy list and have it available in pdf or print form so you can give/send to a new customer. 

Describe the type cakes you do (fondant only, buttercream only, mixed media, tiered, no licensed characters, etc.), your payment and delivery policy as well as minimum purchase, cake tasting options, etc.

This way everyone can be clear and you have documentation that you've provided this information.

Back to the cake...indeed you have underpriced.  Sit down and decided a minimum price for a basic layer cake, a two or three tiered cake, sugar flowers, etc., and spell it all out in your policies.  It will save you a lot of headaches later on!

I have started offering either a sample box of 4 various cupcakes, baker's choice, for $10.00 or they can order a dozen cupcakes, one flavor per dozen at regular price if they want a specific flavor. Baker's choice means I scoop one cupcake worth of batter from each flavor cake i do that week. Or, if I dont have any batters on hand that week I just make a 1/4 batch of batter and freeze the extra cupcakes. If I do that, I do basic flavors, 2 vanilla, one chocolate, one red velvet. Put a different frosting or filling on the 2 vanilla. Voila! I also make a half batch of frosting and divide it into 4 bowls. Add cocoa to one, wedding cake flavor to one, coffee to one, etc. Freeze the leftover. Hopefully I'll have another tasting withing the month and can use the frozen stuff. If not, you can package the leftover sample boxes and add a menu and business card and deliver it to prospective clients for advertising. (Daycare centers, wedding halls, planners, florists, etc.)