Han, you cannot be driven bewildered because being bewildered is a state of being perplexed and confused; very puzzled. You can be driven crazy or driven up the wall, but never driven bewildered. There is no such thing as an unknown charge on a credit card because the store where the transaction was made is always indicated. That transaction should instead be referenced as a misposting or mistaken posting on your card. There is a confusing reference in your first paragraph:
The transaction accumulated to the total I'd used so as a result,
- Did you mean to say that the transaction forced you to go over your credit card limit instead? I'm not entirely sure what you meant to say here. Remember, don't thinkin your language then translate to English. That is how you end up with confusingly worded sentences such as the above example. Think in English, write in English.
You do not ask the credit card company to discharge the $50, you ask them to deduct it. Deduct means to remove from a balance while discharge means to tell (someone) officially that they can or must leave a place or situation. You should not be saying you refuse to pay but rather that you hope an investigation as to how the error occurred and that you will cooperate with them. By the way, you don't need to explain to them that you went to PetSmart with a friend but did not buy anything. That is irrelevant to the misposting.
While you do have several errors in the presentation of this letter, you did manage to present all the required information and expectations as to how the problem can be resolved. If your grammar issues are not considered, this letter is somewhat acceptable in presentation.