Products sold by length

Hi,
I cannot figure out how to set up products sold by length.

quick help, please.

thanks.

You would need to modify lib_cart.inc.php to multiply the price with the value from an option.

extra detail help will be really appreciate.
not sure how to modify and what modify inside the file.
Thanks.

I'm sorry but that could be an hour of work and the forum guideline is that we gladely point you in the right directions, but we recommend hiring someone doing work for you.