I ordered something about a month ago and was able to pay by credit card- they sent me a link to pay via email afterwards. Unless they have since removed that option, maybe you can email them to see if it's still possible?
bacteriostatic saline is better for mixing as there is a preservative (I would mix and let sit at least 72h) in it and it won't burn/sting when you inject. There is no performance difference for water vs saline. If you use water make sure it is bacteriostatic and not just sterile.
I also have AS and take celebrex 2x day for years with no issues. It is a cox-2 inhibitor whereas most other NSAIDS are non-specific COX inhibitors and have a broader spectrum of activity although neither have been measured at the 'skin or dermal level' that I know of. From personal experience I can't say it has affected anything I've had done- MN, sculptra. threads etc. You should be ok. The good thing with celebrex is it does not affect clotting so you will not see an increased risk of bruising like with other NSAIDS.