Lisa Beil began carrying on her business in a GST province on July 1, 2020 and has a calendar fiscal year end. Lisa registered for annual GST filing on May 1, 2021. Today is April 30, 2022, and Lisa just realized she was required to charge GST as of February 1, 2021 since her taxable supplies exceeded $30,000 under the cumulative sales test during December 2020. Lisa billed customers $15,000 exclusive of any applicable GST between February 1 and April 30, 2021. 

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