This is PageFly - a Free Landing Page Builder for more than 90000 Shopify merchants, I would love to provide a solution to your question.
In its settings, you can choose the price for the extra options.
Especially, PageFly integrates with Zepto, you can not only build a beautiful, high-converting store with PageFly but also use these apps to customize your products.
Let me know if you have any further questions. Like my answer, if you find it helpful.
There are many apps that offer this solution. Search the Shopify app store and shop for the best one for your needs. : )🙂
Note: some apps automatically inject theme code when you install the app. It's always best practice to duplicate your theme prior to app installation, especially for new, less established apps.
You can use the Inkybay - Product Customizer app to add personalization to the t-shirt with an additional fee as you want. You can define your personalization fee based on a number of print locations (like Front, Back, Sleeves). The app will automatically calculate and display the total cost (including personalization) while customers designing their products.