Lisa began carrying on her business in a GST province on July 1, 20X5 and has a calendar fiscal year end. Lisa registered for GST on May 1, 20X6. Today is April 30, 20X7, and Lisa just realized she was required to charge GST as of January 1 since her taxable supplies exceeded $30,000 under the cumulative sales test during December 20X6. Lisa billed customers $15,000 exclusive of any applicable GST between January and April 20X6.

Required:

Advise Lisa on what she should do to correct her situation.