they will let you have a Birkin?
No, there isn't. Generally at most stores, if you demonstrate that you are a loyal customer that appreciates the entire brand -- not just birkins or kellys -- then you're more likely to get offered the bag you want.
In thailand, you have to spend about 300,000 bath (around 7000 us dollar) before you can special order a birkin. i know this becasue i talked to the SA at hermes.But i think if there is any birkin available on the floor,you can just buy it right away.
i think it just depends on your luck w/ the store..
No rule written in stone. Any Birkin on the shelf is first come, first served. You can be a complete stranger or a loyal customer of several years, as long as you're first to complete the sale.
If you want to order, though, it's totally at the discretion of the SAs. SAs are more likely to offer you the chance to request a special order if you have bought other bags before you place your request, but some people have lucked out and bought relatively inexpensive items first or nothing at all, just hit it off with the right SA.