Those "tune-up" places aren't worth it unless you have no idea how to turn a wrench IMHO. In addition to these tune-up shops, the dealerships also charge a hefty price for such "rountine tune-ups."
Also, your 99 has a Coil-On-Plug (COP) ignition, so there are no old-school sparkplug wires per se, if you take a glance under the hood.

Changing the plugs on these engines is very easy to do (make sure the engine is completely cold though, because of the aluminum!). Just remove the spark plug cover plate on the top of the valve cover and you will have access to the coils & the sparkplugs...it's very straightforward.
A lot of folks will run Autolite copper plugs - copper won't last as long as the factory platinum plugs though. Many racers prefer using a copper plug and they are relatively are fairly inexpensive, so changing them more often than a "100,000 mile platinum plug" is no big deal.
Other popular sparkplug choices are Motorcraft, Denso Iridiums, NGK, etc. Be warned though, some of the sparkplugs out there cost a pretty penny.
Changing the oil yourself is probably one of the more easier tasks as well - save your money & perform the tune-up yourself. While you're at it, you could change the fuel filter too - at 50K miles, it's probably a good idea.
The following websites are great resources, Joe Lynch & Shane's (TXSnakecharmer) pages. I suggest you bookmark them and take the time to read through them. You will find plenty of info & lots of "how-tos" on how to work on your '99:
http://www.lx.net/jlynch/
http://www.shane.roberts.net/cobra.htm
G'luck!
