Is there a guarantee on the bug fix?

Yes. If the exact bug we fixed comes back under the same conditions, we re-fix it at no charge. We root-cause the real problem rather than patching the symptom, regression-test cart, checkout, search and product pages before publishing, and hand you a short write-up of what changed. This is the Shopify hub in a wider emergency e-commerce bug fixing service, for theme-specific work see Shopify theme & Liquid bug fixing.