yes of courYes of course they've been ripping you off - but you'll just have to suck it up now
No point in worrying about the exhaust until its fitted - and remember things like brakes & exhausts are wear and tear items and unless your insurance specifically says only OEM parts can be used (which to my knowledge no policy says) they do not count as modifications - you have simply replaced them with what was available at the time. The only things which should be declared are non-standard body kits, spacers and the like - and changes to engine or re-maps