Never give discount to get work.. only give discounts for loyal/repeating clients.. and then only small amounts.
As for Quote vs Invoice.. Id be tempted to say that is wrong and probably illegal to try and invoice for more than the quote.. unless the client has broken your terms and conditions. A quote is just that, it is a quote to say how much the work will cost.. you can't go and say "Actually it was 150% more".. if someone tried to do that to me then i'd tell them to piss off and that I wasn't paying more than the original quote.
Think about going into a shop and seeing an item for $1000 and then going to checkout and them saying "This is now $1500 because our rent went up".. wont happen, the price is the price.
If the client asks for extra work or changes, then you supply them with another quote saying it will cost how ever much extra.
I worry about some of the people of this forum with the way they say they run their business or do their work.