Shipping for the first time using eBay shipping.

[evianmodel]

I sold an item on ebay and am using the shipping service/assistance through ebay.  I entered the box dimensions and weight but the cost looks lower than the same size box and item that I took to the post office last week and shipped out.  I understand shipping to different zones changes the rate but I don't want my buyer to have to deal with a single thing and I want my package to have adequate postage on it.  The dimensions are the dimensions and I even went over on weight by almost a pound to be safe.  Do I trust what the shipping calculation is and just print the shipping label and schedule a pickup, or ship on my own through the postal service and update the purchase with shipped and the tracking number?