annual sales less than 20k, it's consider small supplier. Small supplier does not need to apply GST/QST account.
even if you didn't register GST/QST, it's not consider a serious offense. Basically, you didn't charge the client GST/QST, then you do not need to remit it to the Government.
you also can go through collection agency. Or keep on calling them for the payment. or keep on billing them interest and penalty for late payment