Hi, the manipulation of the products price by end customers is out of the scope of the component.

The only way to add a custom amount that i can think of is the one you already adopted using options.

The price calculation is completely done in PHP server side, there is no way to add a custom field to change price unless the code of the component is heavily customized. As of now i never attempted this so i have not detailed instructions about how to do this.

If you are interested to discuss this further for a custom development contact me by email.