In most cases, customers who place larger orders are better customers.
Many stores give some sort of discount incentive for larger orders. Usually it's a coupon or free shipping.
Though discounting might train the customer to expect discounts in the future and can be harmful to some premium brands.
Another idea is to give the customer something of value for free. A bonus.
It's effectively the same thing for you but it can have a stronger psychological feel to the customer. It's the difference between only paying less VS getting something unexpected.
The natural response is to use the order amount for this criteria.
Don't just limit yourself to that tough. You can also give the incentive for orders that bought a specific quantity or a specific product.
I'm sure there are Shopify apps to do this for you or you can use Shopify Scripts if you're on Shopify Plus, but this can be done by anyone by hand. Just watch your orders for the specific criteria and then add the bonus to the order.
The data nerd in me would say to do that when the order value is higher than your Average Order Value but you can just wing it too.
With the right incentive, this could even turn into a regular process to boost your repeat customers.
If you had Repeat Customer Insights installed, you'd be able to see how this simple process could improve your repeat customer metrics for both new and returning customers.
Get a snapshot of your customer behavior
The sooner you signup for Repeat Customer Insights, the more comparison points your store will have. Each 1st of a the month a new snapshot is taken and saved to your account for analysis.