I too was concerned about Insurance Companies and add ons.
I have insured via a broker for several years now with RSA ( Royal Sun Alliance) Being a retired 'copper' and having had the benefit of driver training I am assured I get a good deal. I have found that RSA have always been OK with queries. I have a protected no claims
BUT when I have been in the position of need they have been excellent.
When we bought the Jazz, they were contacted re change of vehicle and basically it meant no difference from our previous car a V6 Rover 45. However, after a dink in a car park several years back, my Wife was pleased when I fitted front parking sensors to the Rover ( the Rover already had rear ones ) With the Rover, insurance wise, no problems.
I then read on Club Jazz about difficulty with Insurers over add ons, so I asked my broker ( now wholly owned by my compatriots both serving and retired) and asked about fitting front and rear parking sensors. Reply NO problems. No question of extra charge either. Incidentally, never having had a secondhand car with a 2 year warranty before I asked the Honda dealer the question. If I fit front and rear parking sensors myself any problems with warranty? No problems, only caveat was, fit front parking sensors yourself? Well I didn't want to pay Honda about £1000 for the privilege. It may seem like a bit of overkill but a few minutes asking can save a lot of problems later.
I suppose, if you can get a good deal with cheaper insurance then there is always a question of how they do it cheaper, so I suppose they the insurers have to claw the money back somehow. I in my serving days remember no end of problems with cheap insurance and also the stupid berks, who didn't bother to check first finding the Insurers had pulled the plug in THEIR hour of need. In the case of Insurance never take anything for granted.