How to display 'from $xx' price when I don't have native Shopify variants

[WillyMason]

Hi guys,

I use a custom product builder app that allows me to adjust the product features which in turn adjusts the price by adding another product to the cart. But that all happens from the product page onwards. Because I am not using the Shopify variants, it just displays the one price and no 'from $xx' price which is crucial for my customers to know as they click on and proceed to customise a product. 

How would I force a certain page or listing to display 'from $xx' pricing to display on product listing pages without native Shopify variants?

As a bit of context I use the Flex Theme from Out Of The Sandbox. Not sure if this is relevant. 

Cheers in advance!