unit price and options

[briq]

I have set up a website selling ropes and those are often sold per meter which I have set as the unit price for the products.

 

Now I need to have optional accessories attached to the products and I don't know how to do that. The accessories are product customization, costing extra.

 

When a product is sold by piece it's no problem since I can use attributes for the options. However when products are sold per meter then the cost of the option should only be counted once not for every meter.

 

Any ideas how can I solve this?