That is a great deal on what should be a great battery. Never heard of the dealer though, so somebody needs to be a guinea pig and let us know if the vendor can be trusted. There have been too many characters involved in EV's for me to trust anyone right away. I wish ping would offer a 5c cell at a price like that.
You might have gotten away with a 15 ah pack ok, but you are going to have a better chance of having the thing last the bigger it is. You'll be pulling about 1c so that is still ideal for battery longevity.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.