To redeem a promo code, please go to Settings > Redeem Promo Code under the Usage & Billing section. Once you redeem the code there, you can keep track of the available balance by going to Usage & Payments > Receipts > Update Billing.
To redeem a promo code, please go to Settings > Redeem Promo Code under the Usage & Billing section. Once you redeem the code there, you can keep track of the available balance by going to Usage & Payments > Receipts > Update Billing.