Currently, the checkout system is not capable of reserving inventory as items are added to the cart. Your cart is reserved once payment information is confirmed, which does mean that inventory can sell out while you enter payment info. We understand this is frustrating, and we are working on building up enough inventory, particularly for new product launches, to prevent this from being an issue. It's difficult to predict how much we need to produce on first launches since this is still a small, but VERY quickly growing business!