Example VPS billing method

-- ************************************ SUBSCRIPTION BILLING ********************************************
function getChargeAmount(p)
    local chargeAmount = p.billingComp:getBillingValue("charge_amount")
    if(chargeAmount ~= nil) then
        local chargeValue = chargeAmount:getValue()
        if(chargeValue ~= nil) then
            return tonumber(chargeValue)
        end
    end
    return nil
end
function advanceSubscriptionBilling(p)
    if (p == nil) then
        return {
            ref="_advance_subscription_billing",
            name = "Advance Subscription Billing",
            description = "Subscription billing which will charge a set amount at the start of each billing period. If the subscription is cancelled you will be refunded the outstanding amount.",
            permittedPCT = {"PCT_CUST_CREDIT"},
            configuredValues = {
                {
                    key = "charge_amount",
                    name = "Charge amount",
                    description = "The amount that will be charged at the start of each billing period",
                    validator = {validatorType = "NUMERIC_DOUBLE"},
                    measureType = "NUMERIC"
                },
            },
            api = "BILLING",
            version = 1
        }
    end
    if(p.lastBillingRun) then
        local amount = getChargeAmount(p)
        local refundAmount = amount - (amount * p.billingFactor)
        if(refundAmount >= 0.01) then
            return {{charge = refundAmount * -1 , description = "Subscription Refund" }}
        end
    else
        local amount = getChargeAmount(p)
        if (amount >= 0.01) then
            return {{charge = amount , description = "Subscription Charge" }}
        end
    end
    return nil
end
-- ************************************ END OF SUBSCRIPTION BILLING ********************************************
function register ()
    return { "advanceSubscriptionBilling" }
end