You are totally right, using gift cards to issue store credit is the most accurate way to go. AltoQ Exchange automates this process by processing all scenarios of exchanges even and uneven within Shopify, and of course issuing store credit. We offer a 30 day free trial for you to assess confidently if this solution works for you. Feel free to reach out anytime - email@example.com
Beware any solution that uses discounts or discount codes. It will understate your sales income.