Found this forum while searching for answers/studies on this, unable to find any discussion/answers online.
The website I deal with is an online car parts retailer that sells in UK and Ireland mostly.
We currently offer free shipping on all orders over €20 and the average order value is roughly €60.
I believe this model of free shipping is unsustainable in the long run due to rising prices. I would like to bring in priority shipping similar to the way Amazon/ASOS do it, where the customer pays ~€5 for a 2 day shipping service and can have the product delivered for free if they are willing to wait 3-5 days.