How to contact Apple for the Mac store billing support?
It's a three step process - with pictures below!


Click support in the Quick Links tab
on the right hand side of the App
Store app.
Click billing in the web page
that opened as a result of step 1.
Feed in all the information and wait
the 24 hours or so it takes to get
an initial response.


It is so worth it to ask for a refund or help (especially in your case where you didn't intend to buy yet) - everyone wins in the end as developers and Apple really don't want a sale that ends up with an unsatisfied customer. 

The system is not perfect nor is it set up to allow previews of purchases, so no one should feel bad requesting a refund or two. Especially so if the program wasn't explained well or is seriously faulty. Would you want a $ and change if it made someone have a lasting bad impression of the work for which you get paid?

Do feel bad if you find yourself abusing the system or on your 10th refund request with no good reason and just not getting the picture.