Actually, it's not just the booklets. The same is true of the coils as well. I'm guessing that the only way to order the BCA flags is to supply the BCA item number (ends in 02 rather than 04) when ordering via phone.
@JLLebbert. You are correct. I called Kansas today and it turned out that the BCA booklets are in fact available to order. They couldn't give me an explanation as to why the BCA booklets are no longer listed in the catalog or on the online store.
Why would the USPS limit sales of BCA booklets to phone orders? It is so nonsensical.