Some things last longer...
You want use your iPhone for phoning or surfing in the web without limits. Horrible situation if now your battery is weak.
But never mind, you can buy this new replacement battery from Sintech for your iPhone 3GS. The set includes the battery and tools for opening the iPhone.
Please note that this battery is usable only in iPhone 3GS, not the old iPhone, iPhone 3G or iPhone 4.