If you are buying a large volume of stamps (eg. for wedding invitations) it might be worth searching for some on discount. This can be tricky and there are not many hard and fast rules you can follow to get a discount. The most consistent form of discounted stamps that I can find is on Ebay. Search for postage stamp rolls and its quite easy to get a 10% or so discount off what you would pay in store for a roll of stamps.
Head to Ebay now and have a look.