Yeah, definitely some good advice there Hua. The full face is meant more for fast, unpredictable, high impact situations. A normal skull cap type helmet will be good enough for normal riding.
If you do intend to save money by purchasing online you can still do so by trying the helmet in a store first like Hua mentioned. Getting it fitted properly and then taking the model number and size to your online shop can save some dollars. It probably won't be much and/or worth the trouble of not just buying it at the local shop.
My girlfriend is purchasing a 22" bmx next week from one of my brothers. It's already half paid for. For anyone that rides bmx that hasn't tried a 22", try to ride one if you see someone that has one at a local park. It's a real game changer. We have a fly out with a huge roll in at some of the local jumps I ride and I was getting probably 4' off the ground pretty easily on my bike. I hopped on my brother's 22" that I am not used to and have only ridden a few times and instantly went like 6.5' into the air without effort. My brother and i were both pretty amazed that I got that much higher on his bike without even trying.