US stamps from the 1940s to date, except for some special issues, can still be bought for face or less. As a collector, I've bought them at 50-60% of face value. The larger denominations, from when postage was 30 cents or greater are worth more. The older 3 cent, 4 cent etc are discounted the heaviest since you need a ton of them to make the 55 cent current mail rate!
Your shipper probably buys postage from my sources!