Lists your subscriptions together with the monthly add-on fee that applies to each. Only subscriptions that are active, live, paused, inactive or future are included; omitting customerId aggregates every billing customer on your account. The add-on is chosen by your account's language — English or French — and by isTollFree, so an account set to any other language gets an empty result. The fee is the highest matching add-on price on the subscription, falling back to the catalog price, or zero if the catalog cannot be read.