With pay-as-you-go there is no billing date: you're only charged when you add credits, or when Auto Top-Up adds your chosen amount because the balance dropped below $5 (at most once per day).
If you're on one of the older monthly or yearly top-ups, the Subscription page shows your recurring amount and the next top-up date at the top. This is the date when your next top-up will be charged.

On your billing date, Storrito automatically charges your payment method for your recurring top-up amount. Your wallet is then topped up with the corresponding credits.
If your payment fails, your subscription moves to a past-due state — see Past Due Subscriptions for troubleshooting.
After each successful charge, your invoice is available on the Subscription page. Click Invoices next to your recent transactions to see your billing history or download individual invoices.