If you copy the button code we provide in Seller Admin and paste that into your own Web site where you want to sell the product (rather than posting product listing to your E-junkie Shop page), you can add whatever explanatory text you wish, wherever you wish in your own page.
If you don't have your own Web site and prefer to post a product listing to your E-junkie Shop (as with the link you provided), the button code cannot be customized on that page, so you would need to add explanation in the product's Details, which appears above the fields and Add to Cart button.
Items using "Variations which tell more about the product" can only accept up to 200 characters for the first option field, and 98 characters for the second and third option fields. Even if you could modify the fields to accept more characters, PayPal would just discard any extra characters which exceed those limits anyway.
You can add custom HTML to your E-junkie-generated thank-you pages, such as adding an HTML contact form with a text field that submits directly to your email address. This help page explains more about customizing your E-junkie thank-you pages:
If you go that route, you should probably customize your Common Thank-you Email template (in Seller Admin > Account Preferences) to explain that that buyers should visit the [%thankyou_link%] provided there to submit their info, if they did not already do so immediately following checkout.