What size bb's should I use with my JG BAR 10 spring powered sniper rifle?

I have an AEG Rifle that uses .2g's so i hav tons of biodegradable .2g's. I am about to order a JG BAR 10 scope package for $120 but need to spend $30 more for free shipping. I hav never sniped before so i'm getting this gun to see if i like it. If i do like it I am planning on upgrading it to get a 500+ fps and better accuracy. However, I have never customized a gun before so I need some help. First of all, before I upgrade this gun, what size bb's should i use? second of all, what should i spend the $30+ on to get the free shipping? I'm ordering from www.hitguns.com so they have lots of different mods and accessories to choose from. I was hoping there was an upgrade that i could get that will work with the rest of the gun having stock parts, like a new barrel. I'm willing to spend more than the $30, but i just don't want to go all out on this gun without even knowing if I will like it. Last question is about scopes. there is way too many to choose from so that's why i thought I would just get the stock scope for only $20 more. If you have any other suggestion on scopes please tell.