I'm in the same situation with PRA.... Just submitted my strike though list today. I just reread the paper, looks like it didn't define a due date other than the bottom print, payment due upon receipt... Weird not to include a date, what did the arbs schedule say? did it provide a time period? JAMS time period?
Texas Justice Court does not require evidence be attached to the complaint. It also requires the permission of the court to discovery to get what evidence they intend to use. You need to decide what strategy you intend to use. Synchrony has the BEST arbitration clause of all the creditors. I would recommend answering with a general denial and invoking the affirmative defense of private contractual arbitration. At the same time you file the answer I would file a motion to compel arbitration. Make sure you serve copies of both on the opposing attorney.
There are threads on this site that can give you an example of both to modify to your facts and submit to the court.