I'm not really sure what your question is, but I'll say this:
You can use the eBay shipping calculator http://www.ebay.com/shp/Calculator to get FedEx shipping costs.
It produces the same prices* as the calculator in listings that buyers use, and accounts for any discounts you may be entitled to. It always produces accurate numbers for me (within a few cents - difference is due to FedEx fuel surcharges changing in the time between when I use the calculator to estimate and when an item actually sells - can be months)
In spite of the misinformation many post here about the eBay calculators and FedEx, the calculators DO incorporate Delivery Area Surcharges (DAS), Fuel Surcharges, and Residential surcharges. My billed amounts are always the same as calculated (with 1 exception 3 years ago that was likely due to a mis-weighing on the FedEx package conveyor).
However, you need to be familiar with how FedEx shipping works to avoid gotchas. The eBay FedEx calculator is simplistic and doesn't handle all the possible surcharge situations like a FedEx Ground package that is 49" long and thereby incurs an $11 Additional Handling Surcharge.
You absolutely must download the FedEx Service Guide from the FedEx website (google for it to get a link to the pdf), and read and understand it - especially the 15 or so pages near the end of the document pertaining to "Fees and Other Shipping Information" - that is where the details are.
Search these forums for old threads pertaining to setting things up to get an eBay FedEx discount - it's worth it - to the tune of up to 25% for some shipments. It would be a good idea to have your own outside FedEx account also in case you are one of the unlucky ones that can't get billed correctly for FedEx through eBay.
*I think there may be a recent issue (starting this year) with the eBay calculators not handling FedEx shipments where dimensional weight is a large factor. I believe that eBay is incorrectly using last year's divisor in the dimensional weight calculations some or all of the time (dividing by 166 instead of 139 which is the current correct value). I started a thread about it a couple of weeks ago, nobody seemed interested, and I have not had time to run tests/scenarios to obtain proof and a rationale for how it's failing. http://community.ebay.com/t5/Shipping-Returns/FedEx-Dimensional-Weight-Calculation-Error-in-eBay-Lab...
A way to get a handle on that issue is to use the eBay calculator to estimate an amount BEFORE listing an item (say to a zone 8 destination), manually calculate the base rate using the rates in the 2017 FedEx Service Guide, and compare them. If a large discrepancy, then work backwards and see if it is due to eBay calculating the wrong dimensional weight (too low because they used L x W x H / 166 instead of L x W x H / 139.) If so, be aware that your buyer will then be charged approx 10% -20% under the actual label cost and you will be eating that amount.
The eBay calculator referenced at the top of this post does not provide a breakdown of the FedEx amounts. If trying to compare rates calculated manually to what eBay is doing you need to fake up a shipment using the actual eBay labels page by selecting something from your sold items (but not yet delivered), choosing to create another label for the item, and changing the address, weight, size, shipper, etc to match your test criteria and see what it comes up with as an amount (FedEx transportation costs are broken down and can be accessed by clicking the little arrow next to the cost on the upper right). Then just cancel or close the page.
Another note: ALL packages shipped via FedEx and UPS are subject to dimensional weight now. There is no 1 cu-ft exemption like USPS has anymore. In most "normal" case for small packages that aren't full of nothing but air it isn't an issue, but a box with something like a cowboy hat in it is going to bite you if you aren't careful.
In summary, FedEx works well with eBay most of the time, but there can be some issues, you really can't treat it as total "easybutton" and assume pushing buttons blindly on eBay is going to get the result you think, you need to read the FedEx service guide, and learn that FedEx (and UPS) unlike USPS*, can have after shipment surcharges. That's why you need to read and understand the fees information in the guide.)
*USPS DOES have some after the fact surcharges - when the shipper screws up the input data (under weight or wrong mail class) the bill increases when "postage due" is assessed. Same idea for FedEx. Additionally some of the conditions that incur surcharges (too long etc) are not handled at all by eBay shipping calculator, and other (like a bad delivery address, a label that falls off or is damaged) are things that occur after shipping and can't be estimated beforehand.
