I believe the inventory in Wix store product list ( www.wix.com/dashboard/*********/store/products ) needs improvement: one more column in addition to "inventory" column that shows the quantity.
The additional column must show the reserved items: the items that have been ordered online but have not been shipped to the customer yet. These items must be deducted from the available qty immediately upon placing the online order and updated on the website as "out of stock", if the total qty becomes 0.
Otherwise I have this situation: for example, I have 1 item available on website. First client orders this item but the inventory is not updated until I ship the item and press "fulfilled" button. Before I ship the item the website will still be showing 1 item available, which is not true - it is already reserved by the first client (but not shipped yet). So a second client can order the same item in this time period (time between the placed order and shipment may take few days) and in this situation I have two orders for only 1 item!