There does not appear to be any official statement released, but you can read the shipping information here or when you head to checkout.
Orders including $25 or more of eligible items qualify for FREE Shipping. With free shipping, your order will be delivered 5-10 business days after all of your items are available to ship, including pre-order items.
You will receive your credit after your order ships. All you need to do is simply fill up your Prime Pantry box with items you want to purchase, and we'll automatically apply the credit at checkout. Limit one $5 credit per one Prime Pantry order. Plus keep in mind that when you purchase 5 Prime Pantry items you will receive free shipping.