There are no specific bulk discounts for using Accredible. Larger plans will allow you to issue more credentials at a cheaper unit cost.
You can get a discount if you have a discount code, which you enter at the time of purchase. There will be an option in your check-out process to enter a discount code which might correspond to a promotion Accredible is holding for a limited time. You won't be able to apply a discount code retro-actively, unfortunately. If you received a promo code in your email from Accredible, it may be a good time to consider a plan upgrade or feature add-on!