Welcome to SCF, Kevin - and I forgive you for your interest in Bahawalpur

Bahawalpur was a rather special case. Gibbons lists it after Pakistan, not India, as it was the only Indian State issuing stamps at the time that acceded to Pakistan. (Las Bela akso acceded to Pakistan, but it only issued stamps between 1897 and 1907.) You might have to look under Pakistan to find it.
Now, as to the stamps. Like those of most of the Indian States, they were only valid for postage within Bahawalpur itself, possibly barring the period from August to October 1947, between the end of British rule in India and Bahawalpur joining Pakistan. (Their status at that time is uncertain.) In any case, Bahawalpur never was, nor ever could be, a member of the UPU.
The stamps themselves are a bit tangled. However, we can divide them into four periods:
(1) 1933: Bahwalpur tried unsuccessfully to get the Imperial Post Office to agree to allow it to run its own postal service. Bahwalpur produced a stamp in
anticipation of the go-ahead, but it was never officially authorised, although a few copies seem to have passed through the mail. This is the listing in the old Bridger & Kay 5 Reigns Catalogue:

(2)1945: The British gave approval for Bahwalpur to issue stamps for government mail (SERVICE) only. These consisted of Revenue stamps overprinted in Urdu 'SARKARI' ("SERVICE"). Some of these are rather scarce:

They
were certainly legitimately used on mail, although in most cases, used are scarcer than mint:

(3) 15 August to 3 October 1947: The overprints on India. These are scarce to rare mint, and much worse used. Gibbons prices all values from ¼ Anna to 12 Annas at £38 each, which is pretty fair. You won't get a
genuine one much cheaper.
(4) 1947: After joining Pakistan, Bahawalpur opened its post office to the general public of Bahawalpur.
Period 1 (1933) is your choice: the stamps turn up occasionally, and sell at around $50 each mint. Genuinely used you can forget about: there are fewer than five in existence.
Period 2 (1945) are - to my mind - thoroughly legitimate. If I collected Bahawalpur seriously, these are the ones I'd focus on.
Period 3 (1947) Good luck, but you'll need very deep pockets (Gibbons' price for the set is £10,000) and a lot of patience

Period 4 (1947) These are next to wallpaper. Some of them were used legitimately, but the great majority of used are CTO. The first pictorial definitives were probably fairly
halal when they first came out. However, the story goes that the Nawab was fond of gambling on the horses when in London. When he ran short of cash, it is said, he would phone up De La Rues and order another printing of his stamps - with the result that the reissued high values in different colours of 1948 still sell at new issue prices. (The 10 Rupee Service overprint is listed at 80p in Gibbons, or about two times face value in 1948.) The UPU commemoratives were pure exploitation, since of course, Bahawalpur wasn't a member.
In defence of the Nawab, it has to be said that he was a keen stamp collector. That's why his stamps are so good. It's just a pity he debased them. Of course, every collector is free to collect whatever they want ... but
I don't take Bahwalpur seriously.
I
do take the other Pakistani State, Las Bela, seriously though:
