By adding an UploadKit field to your product and targeting a specific variant, you can create add-on pricing for custom printing.
To do this, create variant options for your product inside Shopify - for this product we've added an option called Custom logo printing with the values Yes (+$2.00) and No:
You can then set your custom per-variant pricing, and inside UploadKit you create your field as normal, targeting your product(s). Next, under 'Product targeting' in your field settings, set it to Show on variants matching any of these conditions and enter the conditions:
UploadKit will automatically show and hide the field when the appropriate variant(s) are selected on the product page, and your theme will adjust the pricing accordingly.
And of course, if your field is set to 'required', it will only be required if the relevant variants are selected on the product page.