This is slightly unbeknownst to me. In WP8 store, if purchasing consumables, the client has to report fulfillment. Is this when the customer gets billed or do they get billed directly on purchase? i.e. Can the consumables be used like durables if fulfillment is not reported.
The customer gets billed when they purchase. You could choose to leave the IAP unfulfilled if you wanted to, but you'd need to report fulfillment before the customer could purchase that IAP a second time.
If you want to use the IAP as a durable though you should just use a durable. One thing to keep in mind is that if your app is uninstalled, then reinstalled (on same device or different device w/ same Microsoft account) you'll still be able to check for a list of purchased Durable IAPs using the ProductLicenses API. This list will not return info about the Consumable IAPs though, you're expected to fulfill those and track the consumable content through your own means.