Many collectors have this problem with most stamp agencies from most countries. The solution is to buy the "year pack," a single purchase once a year. They can afford to ship you all their single stamps at one time each year.
An alternate, though probably not as cheap, is to buy from dealers on
ebay or Hipstamp. Buying many single stamps, as I've done many many times, makes the shipping cost (which is often as low as a dollar or two) easy to take.
Either approach will get you the single stamps you want. Have fun!