Cloud agents are charged on monthly basis for selected period of months. The more period of subscription you order, the more discount you get.
If you order additional agents while you have active subscription on other agents, the price would be calculated till the period when initial cloud agents expire.
For example: you have 1 agent that expires in 1 month and 5 days and you paid $15/month for it when ordered. You want to add 1 more agent. The price would be calculated based on initial cost ($15/month for each platform), so final price for additional agent would be around $12,5 till the end of your subscription.