How can I get a refund for a payment?

I was charged AUD306.90 today by Dropbox. I may have signed up for a free trial (don't actually recall) but that aside, I did not receive advance notification that my credit card would be charged unless I (a) cancelled and did not proceed with the subscription or (b) agreed and accepted the charge.  I honestly have no recollection of providing my credit card details but I must have some time ago. I don't use Dropbox as the system administrator could attest if they checked my account. Furthermore, I have never been charged before. 

I would like to cancel my supposed subscription and have the charge refunded as billing by stealth is unethical and quite possibly contravenes consumer law in that Dropbox failed to notify me to obtain my consent before processing the charge. I'm sure I'm not the only one who has been caught out this way.

As others have noted there is zero human customer support, which only adds to the frustration! Any assistance would be appreciated.