For example, Wrapin - Gift Wrap & Options adds a checkbox to your chosen product pages where your customer can indicate if they do want the gift bag included in their purchase. Or there are also apps like Terms and Condition Checkbox which appear on the cart page. With this app, you're able to customize the wording of the check box to be applicable to your gift bag option rather than a terms and conditions agreement. To learn more about third party apps please review the Help Docs included in Apps for your Shopify store.
Hope this helps!
You’re welcome, @wonjong!
Which theme are you using? If it’s one of the vintage free Shopify themes, then there is a tutorial for Add a gift wrap option to your cart page. Although this coding is for customers to add a paid gift wrapping option at the cart, you could still use it for your needs by setting the gift wrap option to $0.00. And instead of having customers opt out of the gift bag, they would need to opt in, instead. Does that make sense?
If you’d like help with this tutorial, and you’re using a free Shopify theme, then please create a support request through the Shopify Help Center. A Support Advisor would be happy to check your design time eligibility and escalate it to the Theme department on your behalf. Alternatively, for any third party theme code customizations, I’m afraid I don’t have resources to share for those. However, a Shopify Expert will be able to assist you with it. You can read more about Shopify Experts in the Help Doc Hiring help.
Let me know if you have any more questions!