Normally, the military discounts programs on some occasions of the year. In order to know whether it is offered or not is depended on Overnight Prints (US)'s policy sales, you can visit the store now for your reference.

In case you enter a discount code during checkout that doesn't exist or isn't currently active, then you can see the message: “Unable to find a valid discount matching the code entered”.

To avoid this problem, you should check out carefully Overnight Prints (US) promo code before that promo code could be expired, some promo codes are tied to a minimum purchase, a first-time purchase, or tied to geography, and other reasons.

Usually, Coupon Stacking is not allowed at Overnight Prints (US). However, the store will surely give out notice if available.

Yes, it has ever. It may not be available now, but it is completely possible for loyal customers of Overnight Prints (US) to get it by tracking all of its promotion programs.