usps sells insurance for their packages. If you buy more than $50 worth, it will have a number that can be used to verify delivery, but you can't "track" packages the way you can with Fedex or UPS, where you can see where it is along the way. With less than $50 insurance, or no insurance, you can buy delivery confirmation.
Probably your buyer wants some proof of when you ship it, and the delvery confirmation ro insurance number will give him somethnig to "track" online.
btw, since this is probably a small package, it probably would be almost as cheap to send it first class or priority, wouldn't it ? It will get there much faster.