I mainly use UPS or USPS, but the online calculators almost always differ from the actual price.
Most expensive is at the local "pack & ship" place, so if at all possible, I hit the local UPS store & PO with the package before quoting the buyer.
A couple of times I've shipped something cheaper at my nearest PO than the one in town, which would seem impossible.
Even weirder is when the USPS online calculator quotes higher than what that cheaper PO charges me.
In the end, it almost washes out, and I just can't get too bothered by a couple bucks...
Main thing for me nowadays is to clear my shelves, get a reasonable price, and have a happy buyer.