Discounts cannot be applied to flat rate pricing, even if that flat rate pricing includes per ticket pricing for additional guests.

Instead, we recommend setting up a new flat rate scale that already includes the member discount. 

For example, you would have a Birthday Party flat rate and a Member Birthday Party flat rate scale.  The Birthday Party flat rate would cost $350 as an example, and the Member Birthday Party flat rate would cost $315 which takes out a 10% discount.