Some e commerce site calculate GST by add a separate line item to your invoice. The itemise GST amounts are then not passed to PayPal and not subsequently passed to Billx.
We recommend that you ensure your ecommerce site passes the itemises GST amount to PayPal, and not to add GST as a line item.
Billx will pick up line items with the keys words GST or Sales Tax and treat these amounts are your GST amounts.