We're discussing how to have "assembly" as an option when customers buy our products.

Example: Customer buys 4 tyres and 4 rims. Assembly is optional - checkbox, adding price 100 x 4 = 400

How would you go on to implement this? Any suggestions are highly appreciated.

Hello,

What about product attributes with price adjustment? On product you will have the checkbox with "Assembly" which costs 100$. If your customer purchase 4 items, he will have additional 100$ x 4. Maybe this scenario will meet your requirements?