You are able to pay for an invoice from the online portal at my.call-ruby.com. 

STEP 1: Log in to your account at my.call-ruby.com and go to Account, then Billing from the menu on the left. 
STEP 2: From there, navigate to the Invoices tab where you'll see all of your paid and unpaid invoices.
STEP 3: Select the invoice you'd like to pay.
STEP 4: Follow the steps on the screen to complete your payment.