A Sale Order Line has a Subscription ID.

You can navigate from the Sale Order Line to the Subscription to the Subscription Lines, find the matching product and move the commission rate over that way.

Note:  This code sample doesn't do anything apart from look for a matching product.  If you have different rates for the same product on the Sales Order it won't work - this is just meant to get you started.

You can probably create a related field on the Invoice to grab the rate from the Subscription Line.

1. Create a Server Action that you add to the Form View of the Sales Order via a custom BUTTON. 

# confirm the Sales Order
record.action_confirm()

# do we have order lines?
if record.order_line:
  
  # loop through order lines
  for so_line in record.order_line:
  
    #do we have a subscription created and linked to the so line
    if so_line.subscription_id:
      
      #do we have lines on the subscription?
      if so_line.subscription_id.recurring_invoice_line_ids:
        
        #loop through the subscription lines
        for sub_line in so_line.subscription_id.recurring_invoice_line_ids:
  
          #do the products match?
          if so_line.product_id.id == sub_line.product_id.id:
  
            #copy commission
            sub_line['x_commission'] = so_line.x_commission

2. Hide the CONFIRM ORDER button so users use your button instead.