Post paid bill sent 4 hours after i start service????

Ok today I just started Tmo service today, I received my S24U and port my number from ATT, then like 4 hours later I received I bill for 90.00, with the other carriers I been with, I never received a bill until after 30 days of service was used, what is Tmo trying to pull here? I talked to chat reps and they seem clueless, plus the bill is do the 24 th of this month.