When you have more than 30 domain names, you will now take advantage of up to 60% discount on the regular price! It applies when you buy new domain names, when you renew yours or even for domain names transfers!
There are no monthly cost and the discount applies automatically when you pay.
For instance, the .com domain names is only $9,99 instead of $14,99 with the volume discount. .ca or .net are $10,99 instead of $14,99.
You can check the list of TLDs with the discounts here.