Booking.com charging small Canadian business partners for GST on guest payments that they collected.

We are a small Canadian business partner that is not registered for GST. Booking.com has invoiced us for GST (VAT like tax) on guest payments that they collected,  when they should have collected it when they collected the booking cost from the guests. 

I have spoken to the CRA, Canadian Tax Authority, who confirmed the GST should have been collected by Booking.com from the guests not from us, the supplier of the service.

I messaged booking.com almost 2 weeks ago, shortly after they sent me the invoices. To date, they have not deigned to reply to my message and have now threatened to lock our account for non payment of the 2 invoices.

Any suggestion on how to resolve this issue other than capitulating to their demand to pay these incorrect invoices.