If you have disabled the automatic credit option or if a new invoice was manually generated, you are able to manually pay an invoice from your account balance. 

In order to proceed, you would simply need to go to the 'My Invoices' section and click on the invoice you would like to create (even partially) with your account balance. 

Once on your invoice, you'll notice a green section and input which allows you to specify the amount of credit to allocate to the invoice and you may click on apply credit. 

Once the credit applied and if the credit covers the invoice entirely, the Proforma invoice will be automatically marked as paid and you'll be able to download the payment receipt invoice. If the credit doesn't cover the invoice amount, the Proforma invoice's amount will be updated and you'll be able to pay for only the remaining amount.