With Koin, you can set up Expired dates for Credit, which encourages customers to buy again soon to take advantage of their earned amount.
And then, if you want to notify customers about almost-expiring credits, You can use Shopify Segments - Shopify Flow - Shopify Email/Klaviyo altogether to set up this flow.
On Shopify Customers section, create a Customers segment for those ones that have almost-expired store credits.
Shopify create some templates that can be used in this case, such as Customers with store credit expiring in the next 7 days:
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Tips: if you want to create another email flow for customers who has not used Credit in a certain time, you can refer to the template Customers who have not used their store credit after 1 month and create a segment. Other steps are the same.
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Flow description: When customer joins the segment “Customers with store credit expiring in the next 7 days” > send them an email.
At this step, you will need to use Shopify Email or Klaviyo to set up and send the email.