Two rules in Settings > Restocking, in the Automation section, order stock without waiting for you to notice it ran out. One raises a purchase order when an order comes in for something you do not have. The other drafts restock orders for items at or below their reorder point. For items your suppliers ship straight to your customers, see Create dropship purchase orders automatically.
Purchase orders for out-of-stock items
Tick Create purchase orders for out-of-stock items and, when an order comes in for something you do not have, Stockroom raises a purchase order for what you were short of, so you can order it or make it without watching for it.
Only the shortfall. If five are ordered and three were on the shelf, the purchase order is for two - not five. An order that simply cleared the last of your stock creates nothing, because you had enough.
Stock is read at the location the order was routed to, so a multi-location store orders for the place that actually ran short. The purchase order is set to receive back at that same location.
These are ordinary purchase orders. Unlike dropship ones they ship to you, they add incoming stock in Shopify, and you receive them normally.
They share Create when, Skip orders with these tags, Created as and Email the PO to the supplier with the dropship rule, and those settings work with only this rule turned on. You can run either rule on its own, or both.
Two things decide whether this can fire at all. Your store has to allow selling past zero for those items - if Shopify stops the sale when stock runs out, you were never short. And the items have to be tracked in Shopify; products with inventory tracking off have no stock level to be short of, so they are skipped.
One purchase order per order, or one per supplier each day
Purchase orders for out-of-stock items, just under the rule, decides how shortfalls are grouped:
One purchase order per order (the default) - each order gets its own purchase order as it comes in.
One purchase order per supplier each day - through the day, each order's items are added to one draft purchase order per supplier. Choose the Send time beside it, in 15-minute steps, in your store's time zone.
At the send time, each of the day's drafts is marked as ordered if Created as is set to Marked as ordered, and emailed to its supplier if Email the PO to the supplier is on. With Draft for review, they stay drafts for you to check and send. This happens within about 5 minutes of the send time, using those settings as they are at that moment. Orders that come in after the send time go on the next day's purchase order.
A store that runs short at two locations gets one daily draft per supplier for each location, because a purchase order receives at one location.
Dropship purchase orders that ship to your customer are still created one per order, as each order arrives.
Restock drafts: drafting your restocking for you
The same Settings section can also draft your restocking for you. Tick Draft restock orders automatically for low-stock items, then mark suppliers with Auto-draft restock orders on their pages. Any of their items at or below its reorder point is added to a standing draft purchase order for that supplier - one draft per supplier, topped up as more items run low, never a pile of new ones. How often controls the schedule: every night (the default), weekly on Mondays, or monthly on the 1st.
Drafts only. Nothing is ordered, emailed, or sent to a supplier without you - you review the draft, adjust, and mark it as ordered like any other purchase order.
Your own drafts count. If you've already drafted an order covering an item's shortfall, the next run leaves it alone - quantities never double up.
It follows your reorder points. Items use the reorder point and max level from their product page (or Stockroom's suggested thresholds once applied). Items excluded from reorder suggestions are never drafted, and neither are the items of a supplier you've set to Ignore in reorder suggestions on their supplier page.
When something needs you
When the out-of-stock rule cannot finish, a note appears in the Needs your attention feed on Stockroom's Home page, and it clears itself once the problem is fixed. On a store with more than one location, expect this one:
Could not tell which location the order was routed to - only on stores with more than one location, and only for the out-of-stock rule. Stockroom will not guess which location ran short, so create the purchase order from the order's page in Shopify.
Good to know
Turning a rule off stops new purchase orders straight away. The ones already created stay as they are.
Create when, Skip orders with these tags and the 2-day rule for old orders work the same as for the dropship rule - see Choose when and how in Create dropship purchase orders automatically.