Restrictions, shipping and tax settings

Restrictions

Limit the usage of store credit to specific collections and set a minimum/maximum credit threshold that can be used at a time as well as the minimum order purchase amount.

If you want to have different threshold for different customers, then you can set the custom maximim credit threshold dynamically by adding this snippet to your theme.liquid file:

   {% if customer.tags contains "member" %}
      <script>
          window.custom_maximum_credit_usage = 5; // amount
          window.custom_maximum_credit_usage_text = "$5";
          window.custom_minimum_order_purchase_amount = 50; // amount
          window.custom_free_shipping_threshold_value = 100; // amount
     </script>
   {% endif %}

Shipping Settings
You can set the amount of the allowed store credit that will cover the shipping cost if customer has excess store credit - It can be fixed amount only, It is not related to your shipping rules/methods on Shopify.

Tax Settings
You can charge or deduct tax from the store credit that your customer uses i.e. 10%
Note: Taxes and Shipping settings allow fixed amounts only, not related to customers location or order

Credit expiration
Credits expiration resets customer's credit balance to zero after a certain period of inactivity - It doesn't expire one transaction only as they are not individual rewards, but they are transactions that are summed up to a credit balance. 

You can use "{maximum-usage}" tag in the widget settings, It renders the maximum credit usage setting i.e. 10% or $10