Currently, you cannot select a specific variant to include in a bundle step. When you add a product to a bundle step, all of its variants are automatically included.
What This Means
When building a bundle, products are added to a step by product, not by individual variant. For example, if you have a product called "Carrots" with several quantity variants (1kg, 2kg, 5kg), adding "Carrots" to a bundle step will include all of those variants, there's no way to add only one specific variant (like just the 1kg option) while excluding the others.
This applies whether you have "Display variants separately" or "Variant dropdown menu" enabled in your step settings:
Display variants separately: Shows each variant as its own selectable item, but still includes every variant of the product.
Variant dropdown menu: Shows one item per product with a dropdown containing all of that product's variants.
Workaround
If you only want customers to be able to select one specific variant, the current workaround is to create a separate product in Shopify for that specific variant, then add that product to your bundle step instead of the original multi-variant product.
Is This Feature Planned?
This isn't currently supported. If this would be useful for your store, we'd encourage you to submit a feature request through our Community Hub, where our product team reviews and prioritizes requests based on merchant feedback.
If you have any questions, feel free to reach out to our support team.