If you have items that are selling very quickly, like in the case of flash and holiday sales, the inventory syncing between Whiplash and your store can be delayed causing overselling.
After Whiplash imports an order, it will notify your shop of the new quantity. During that import period, your shop may have already accepted payment for new orders that we have not yet imported. So while our system may know the inventory for an item has reached 0, your shop might not get that update until after it's already oversold.
During flash sales, we recommend disabling inventory syncing on the items that are being sold in the sale. This can be done from the item's edit screen under the Rules tab. In the Sync Inventory dropdown, you'll want to change the setting to never:
You can wait to change the dropdown option until after your items arrive at our warehouse. That way your shop has a starting quantity. Disabling the syncing will allow your shop to control the inventory quantity. Since your shop is the one taking the order, it can more reliably prevent overselling. After the sale is over, you can enable syncing again for the item.
Store-wide Flash Sales
If a large part of your catalog is on sale, you can also temporarily disable the inventory sync in the Preferences → General tab: