If I redeem a coupon to a subscription during quantity increase, how much will it discount?

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If you perform a quantity increase on a subscription + redeem a coupon while performing the quantity increase:

Quantity increases result in a charge for the quantity difference, so the coupon discount would apply to the additional quantity that is added during the upgrade, not the entire subscription. 

For example:

Let's say halfway through a monthly billing period subscription to Gold with 5 users is increased to 7 users and each user price is $10. You also decide to add a fixed-price coupon for $10 off.

We get a charge with $0 because:

• 7 Qty - 5 Qty is 2 Qty
• Normal price per user is $10.00
• Proration is 50% due to timing of change
• Price prorated is $10.00 x 50% = $5.00
• $10 off coupon is applied
• 2 Qty x $5.00 Prorated Price - $10 off coupon = $0