That's true but also a reflection of the fact that USPS does not care much for their collectors. I think you can still order singles from SFS, but you will have to pay a $2 custom order charge in addition to $1.25 or $1.75 for p&p if I remember correctly. Hence, I stick to blocks of four now or buy a whole sheet if I like the design.
I don't remember the last time I bought a stamp at the post office. It's just not convenient. You have to wait in line and then get a stamp that has been torn from a sheet carelessly; or it will come with the clerk's fingerprints or has a bent corner. Even philatelic counters are not good (anymore?). We've got one here in Indy, but you're out of luck if you want the "A Flag for all Seasons" stamp booklets and coils from all three printers. Why call it philatelic counter then???
Since USPS now sell on
ebay, I have started to order through
ebay - primarily because you get 2% in
ebay bucks (basically cashback). The downside is that you have to buy whole panes/sheets and some coils in full (who needs 10k of a 1c stamp?). Hot items (i.e. some imperf press sheets which are sold-out before or shortly after the official release date) need to be ordered differently, of course. Here, I mostly use usps.com's online store. But even this store doesn't have everything for completests like me. Hence, I sometimes place a mail order with SFS to get strips from coils and booklets and coils from different printers.
In summary, you need to do a lot of research! I got my own list based on the stamp announcements in the postal bulletin. Of course, you can also subscribe to a philatelic journal.
Overall, philately is a dying hobby. There is no need for the vast number of stamps that are being issued and sometimes even from different printers. Most of the time you mail an item a sticker will be slabbed onto the mail item rather than the clerk assembling the postage using stamps - the label comes at a push of a button, whereas stamps need to be scanned individually, not to mention a bit of math first of all to figure out how many of which face value...
USPS will continue to milk us collectors for sure with high-price stamps such as the Inverted Jenny ($12 a piece!). And judging by the
ebay USPS store sales figures, pockets are still deep...
(I'm sorry for rambling & moving off target - one of those days) :)