FedEx gives estimates, not exact quotes, because there can be surcharges, zip code charges, zone charges, etc. The buyer only has to pay calculated shipping from the listing as far as I know, but if that's wrong, someone will correct me.
Set up your own FedEx account and link it to your eBay account so you can deal directly with FedEx about issues. Otherwise, eBay is the owner of the account you are using to ship to your buyer.
FedEx SmartPost does not have any insurance, and it takes longer for delivery and makes buyers angry. You can't even talk to FedEx SmartPost if there is a shipping issue.
Life shrinks or expands in proportion to one’s courage. ~ Anais Nin