Setup one time service charge without fix cost

I am using version 1.3.9h, I am setting the website to sell special machines. Some machines need customization, so I need to charge a one time service fee. Since each customization is different, one time service fee is different. What is the best way to setup this variable price one time service charge. Basically it works like the Paypal's donate button.

Thanks

What is the best way to setup this variable price one time service charge.
Basically it works like the Paypal's donate button.
Well it will not be like a PP donate here....

If you know or have classes of one time fees that you can pre assign to a specific item then adding it as an attribute would be a way

Base item can be bought stock for XX
Or a customization can be selected and added

Might be that you can have custom level 1,2,3,4....?? each with an increasing added cost

See adding attributes and the examples are for a more standard use as in color & size but you can define them as you need

https://www.zen-cart.com/tutorials/index.php?article=57

http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=492

One time membership mod....BUT currently fixed charge.
Could probably be adapted to suit you requirements

Thanks. Good to know I did not miss any feature. I will try both suggestions.