Sell product with an optional service agreement

[jrgrafisk]

Hi

I'm currently setting up a shop to sell used forklifts.

These forklifts can be bought with either a service agreement or not. The price will of course differ on the the options.
http://imgur.com/rUp8P9k

I've until now tackled this problem by creating two different combinations.

http://imgur.com/nWwI8kc

 

But... This requires me to have a quantity of 1 per combination. This make it seem that I have two of these trucks, when that is not the case.

What I would like is an option to buy the truck with an added service agreement - like an add on product.

Any ideas how to do this?

 

Thanks in advance.