I do not know why on earth that the store used bird seed as a substrate! I would get that out of there as soon as possible, and go with either newspaper (black and white pages, not the colorful ad sections.) I arrange mine so that the hide spots and water dish, weigh it down, so that the snakes are not forever going underneath it and defeating its purpose: to act as a cage liner and absorb any liquid urate matter or spiller water.
I am not angry with you, as you only did what that store said to do. but I am not happy that they are spreading incorrect advice like that. Bird seed hulls are not even eaten by the birds themselves that eat the seeds.. that should say something.