After purchasing successfully your subscriptions, please Edit the subscription that you wish to buy more numbers for

1. Click on blue button "Add more numbers"

2. Choose the Country code and relevant number(s) that you want to purchase ---> click any where on the screen to finish the choosing step. 

3. You will see the New number that you have chosen ---> click on "Next: Billing" ---> follow the instructions 

4. Click "OK" then you already finished the purchase