Limiting Backorders on Shopify



I am selling a popular item that is hard to get from my supplier. I have 5 in stock, but I know my supplier could get me another 50 if I asked. I don’t want shopify to sell 100 of the item because my supplier cannot fulfill a purchase order that big.

How do I limit shopify to only sell 55? It looks like I need tradegecko tell shopify my inventory level is 55 and not 5. Then when I have orders for 40… trade gecko will take 5 from inventory and suggest I make a purchase order for 35.