Paisleymoon is correct: if they shipped it FedEx overnight & they actually have a pick up on Saturday's (a lot of places don't because it costs a substantial amount) then you will get it before 3pm on Monday, barring any delays due to weather issues. Otherwise, it would ship Monday for Tuesday delivery, again, by 3pm. I ship & receive via FedEx almost daily :)
Today is Saturday. If you just ordered it, most likely you'll receive it sometime Monday. Did Radio Shack give you a tracking number? That way you may be able to see where your package is on Monday morning.