You can add customizable text-only fields to your products, without letting customers change the placement of the text or any other aspects of your design:

1. Create the design for your product including a text field, for example “Your text here.”
2. Activate the Customizer for your store (find out how to do that here: How can I add the Customizer to my Shopify store? – Help Center | SPOD).
3. Activate the setting to Allow customers to customize this product in the product details page of the product for which you want to offer this option.
4. Beneath this setting (which would usually activate the full Customizer), you will see the option to "Allow only text editing".

Once this is activated, you can choose which text fields in the design will be customizable.

If the option to "Allow only text editing" is enabled, customers will be able to edit only the text field chosen. All other parts of the design will be fixed and un-selectable in the Customizer.