magento 1.9 – Warranty extension available for purchase in the shopping cart

I would like to develop a module allowing the purchase of a guarantee extension of the payment page.

The price represents a percentage of the total products and always refers to the total basket, not to the single product.

I was thinking of creating a puppet product with sku: WARRANTY_EXT and update the price dynamically with an observer: sales_quote_collect_totals_after.

141/5000
Thus, the customer extended the warranty itself to a separate product.

My question is: is this approach correct? Do you recommend others?