Please that is not a friend at all, she is a user,but I can't blame her I blame you for allowing it.I too am a giver and i've been used too many times but not anymore now my favorite word is NO, in fact I say no first and then later say yes if I can.That way your yes will come with conditions she will only be too happy to accept.
It's only when you learn to treat yourself better that they will recognise how to treat you,you don't need friends like that.
I guess I have been lucky so far (knock on wood). Whenever I make cakes for friends and family I give them a super great price and they always try to give me extra!
Like others have said she doesn't sound like that great of a friend! I would chalk that $20 up as a perfect reason to not have to do anymore cakes for her, and if she asks again give her the fair market price for the cake and tell her you will need payment in advance to buy your supplies. Bet she wont want you to make anymore cakes for her!
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