Customer deduting GST from our invoices

Hi all, 

Recently, we have came across that one customer (one of the leading stores in Canada) is actually not paying us the GST when they pay a lump sum on a few of our invoices. I went to the vendor’s portal and checking about the details... only then that I realize that they deducted all the GST form the invoices we sent to them and only paid us the price before GST. 

Can anybody show me how am I going to post this into SAGE as all the invoices we sent had GST included. For example: we sent $200 worth of product with $10 GST but this customer only paid us $200.

Thanm you in advance for any opinion or suggestion!