Hi,

here is a hint that you can follow and probably going to need little customizations:
1- Create product warranty as service.
2- This product is to be linked with a deferred Revenue account in Income account.
3- you can create a deferred Revenue model by going to Accounting, Settings, deferred Revenue models.

this is very important if you have accounting module installed because your client will pay you the whole warranty price but in your financial book, the price should be devided by the number of years/months.. the warranty is covering.

Then you create your sale order and add the product warranty.

you can check this video for more details: https://www.odoo.com/fr_FR/slides/slide/deferred-revenues-recognition-686?fullscreen=1

P.S: don't forget to tick the prorata temporis.
hope this helps.