Since nobody can help me, I did it myself and came up with it. I did it, I found a solution. Thank myself 
Topic summary
Goal: Let customers choose “picture with frame” or “without frame” within a single product for Gelato-fulfilled prints, preferably without adding another app.
Suggestions:
- Third-party options were proposed despite the no-app preference: PickyStory for bundling/upselling (combining products) and Easify Product Options (free plan) to add a selectable option (e.g., radio buttons/switch) and assign it to the product. Multiple screenshots are provided to illustrate the Easify setup.
Clarifications requested: Whether this can be done natively in Shopify or through Gelato without using apps.
Latest updates: The original poster stated they solved the issue but did not share how. Another participant asked them to post their solution.
Status/outcome: No confirmed native Shopify/Gelato method was provided; only app-based approaches are documented. The discussion remains open, pending the OP sharing their solution.