I'm not only fairly new to Notion, but also new to owning a very small Etsy shop where I plan on selling some simple fan art stickers. As you can imagine, my inventory is really simple and this IMS would be way too simple for even medium-sized businesses. Also, I searched for an IMS in this sub and noticed that others were looking for the same solution so I thought I'd share!
The resources that I used are linked so that you can try this out yourself too! I thought that I would never be able to use relations, rollups, or even formulas but I did! If you're a total noob like me, instead of copying & pasting RG's low supply warning code, type it out yourself because it helped me understand how if codes work.
My setup includes links to the Inventory and Receiving & Dispatch databases then embeds those databases below. R&D is filtered where date is within the past week and sorted by descending date. When you change the filter to the past month, you'll notice that I have added "initial stock" in July and referred to the order numbers from the suppliers and those numbers are positive. Whenever a customer orders something (CO # order_number), it's input into R&D as a negative number. The Quantity rollup in the Inventory database takes that sum. Then that sum and the Minimum number is used in the Warning formula, which will update to red when the Quantity is less than or equal to the Minimum you set; to yellow when the Quantity is 10 less than the Minimum; and to green when the Quantity doesn't match either of those if statements.
Besides that, I swapped the Supplier and Warning columns so that's the only thing that doesn't match the screenshot above!