Nick here from Shopify. Really good question!
Natively on Shopify minimum purchase amounts can only be done by wholesale orders.
"You can require that your customers spend at minimum a set amount each time that they make a wholesale order. You can set a minimum order amount for your wholesale store in the Preferences page. For individual customers, you can set the amount from their customer page. A minimum order amount set for a wholesale customer takes precedence over the minimum order amount set for the store."
You can see the help document Shopify has about this with more detail here.
There is an app from the Shopify app store which you could look into that might be able to do what you need. It's called Order Limits (MinMaxify) and will let you set limits on anything from the total dollar amount to the total weight of the products. You can see it here.
Let me know if you think this can work for you.
All the best, Nick