FedEx shipping through eBay labels is offered alongside USPS shipping on the label page. You can switch carriers using the "Carrier" dropdown.
If you set the listing up to offer FedEx shipping only it will default to that at checkout and on the label page. If you offer both USP and FedEx options, the buyer will choose at checkout and the label page will default to that choice.
In any case, you just make sure all the info is correct, print the label as per usual, tape it on the box (I cover all my labels completely with clear tape) or put in an clear stick on envelope (like the USPS customs pouch), and then drop it at a FedEx shipping center, a participating Walgreens, Albertsons, a box store that does FedEx, or give it to a FedEx Ground driver (catch him making a delivery somewhere or jump out at a stoplight 🙂
Just be aware that FedEx (and UPS) is unlike USPS. There CAN be after the fact added surcharges. If your buyer screws their address up or makes delivery difficult (not around to sign on sig conf multiple times, etc) there can be address correction surcharges, re-deliveru attemp surchrges (after 3 attempts), etc.
Another consideration: I personally NEVER use flat rate shipping for things like this - always calculated. Closer buyers pay less shipping (makes it a better total cost to them) and distant buyers pay more.
With FedEx (and UPS), because of unknowns (like the buyer's zipcode) before the sale when you set up the listing, if you use a flat rate shipping cost (not calculated), you can be bitten with a Delivery Area Surcharge (DAS) of $3.45 to $4.40 for FedEx Home Delivery ($3.45 for rural, and $4.40 for BFE)
If calculated shipping they would be charged any applicable DAS at checkout, but if you go flat shipping charge, you eat it (for this particular item, the slim possibility of a rural buyer purchasing and costing you an extra piddly $4 probably isn't an issue, but can be for other low dollar items and might need to be factored into a flat shipping rate)
FedEx isn't a scary monster, but you do have to be more careful with accurate dimensions (dimensional weight), weights, shapes (log, cylindrical,....) packaging types (not CC boxed, shrink wrapped, ...) in order to not be surprised after the fact (there are criteria, and things like Additional handling surcharges can apply).
Other than a couple of billing glitches through a FedEx account over 10 years ago, and one spurious eBay FedEx Smartpost upcharge 3 or 4 years ago (for magically tripling in weight but was credited), I have had zero issues with billing, damage or loss, and my invoiced amounts are always within pennies.
One thing I didn't mention was insurance. FedEx shipping comes with a default $100 of declared value (like ebay TRS/USPS PM Comm+), and more can be purchased at label generation time (same as USPS) if desired.
A 16"x15"x15" 12 pound corrugated box shipment is nowhere near any of the surcharge criteria, and pretty much no different than a USPS Priority Mail shipment (other than the DAS issue), but if you have any questions about packaging, shape, surcharges, ask.
