Ok advice:don`t do it.
:If you do,spare batteries,for flash and camera.
:Memory cards,lots of Gig,if you take 2000 shots you`ll get some keepers.
:Check out the Venue and where they want the after shots taken,try to go at the time of the wedding.Take practice shots.
:Pre Wedding,talk to B+G,find out what they want.
:Google a "Wedding Shot List"
:Use bounce flash or if you have an assistant,reflecters.
:Take lots of pics,I took 2400 at my last wedding.
:Expect the unexpected,nothing goes to plan when emotions are high.
ON`T PANIC,you have to be at ease,the B+G will be nervy enough,if your relaxed,it will make them easier to work with.
:If you can,take some practice shots in the church and other areas.
If I think of anymore,I`ll post them.Good Luck.