I have this website with an Optional product field but the total doesn’t change when it’s selected Reservar | FotoMaton 360
The Total is being updated for me.
Before selecting the option: Screenshot Ofu30TVCc1.png - Droplr
After selection: Screenshot e2lzAZKE6b.png - Droplr
That said, checking the page headers I found that your host is caching the page: Screenshot kPFauhfoUR.png - Droplr
Caching has been known to cause multiple issues and is not intended for payment pages. So I would recommend you to ask your host support to exclude the URL of your form page from cache.