Welcome to Urban Toronto YYZ_M. Which phase are you in?
With respect to the issue you mentioned, unfortunately this happend to us as well (twice):
1) After our initial colour selction appointment, we went home and reviewed the invoice. Everything looked correct, however the math didn't add up. I noticed that their system had listed the upgraded fridge but the associated cost did not get added to the bill. We decided to call and advise them since we knew they'd find it eventually. We had to go back in and sign the revised document.
2) Many months later (probably back in November or December) we were called again indicating that they (the decor centre) had been given incorrect pricing for certain upgrades by Head Office (Fernbrook). Some of the pricing worked against us (i.e. price increase) while others actually worked in our favor (price decrease). I really let them have it since this was not our fault, and appeared to feel quite bad about it. They worked with us to adjust some of the pricing of other items to minimize the overall increase we'd incur. We had also changed our mind about a couple of our selections and were able to make changes. In the end we spent another $300 or so but got about $600 worth of 'upgrades' so we couldn't complain too much.
If I were you, I would complain and see if they can do anything for you.