The payment method is very easy. Suppose you have chosen the monthly Standard plan for 1 agent for 15 USD and your subscription date starts from 1st March. Then on the first month you can pay 15 USD with your credit card and from next month onwards, 15 USD will be automatically deducted from your saved card.
Now, for 1 year and 2 years subscription, we have 10% and 20% discounts respectively. So if you have selected the Standard plan for 2 agents on a yearly basis, then you will get a 10 % discount on the total amount which you need to pay at a time during the time for your subscription. So at a time, here you need to pay 13.5USDx2x12= 324 USD (for 1 year plan).
The same will be applicable for 2 year subscription with 20% discount.