The only thing I've ever really seen that promotional code apply to is membership renewals or upgrades. I've never seen a discount code for a discount off an extension. In any case, all discount codes are targeted to individual II members. Just because there is a code out there, doesn't mean it will apply to your account.
As for the extension, what is your plan for the deposit? When does the deposit expire. When you extend a deposit, the deposit gets additional restrictions to where it will only be able to book in 30 day Flexchange. You may be better to use Eplus instead of paying for an extension.