I try to add a new product_type "product_rental", that's a product customers can subscripe to. Maybe the word "product_service" would be better, but I am not an expert on english so you may feel welcome to correct me. I have added a table product_rental_extra just like product_music_extra. It has at least three extra fields

rental_period in days
price_for_period
billig_period in days

How and where do I collect these fields?

I think I have to modify the files in
"admin\includes\modules\product"

collect_info.php
new_product_preview.php
preview_info.php
update_product.php

but where else am I supposed to make modifications?