>>I wonder if FedEx charged because the shipping container wasn't actually a box?
That would be the (current) $12 additional handling charge - a Rubbermaid container falls afoul of condition 5:
────────────────────────────────────────────────────────
Additional Handling Surcharge Express Package and Ground Services
A surcharge applies to any package that:
(Dimension)
1) measures greater than 48 inches along its longest side;
2) measures greater than 30 inches along its second-longest side;
(Weight)
3) has an actual weight greater than 70 lbs.;
(Packaging)
4) is not fully encased in an outer shipping container;
5) is encased in an outer shipping container not made of corrugated fiberboard (cardboard) materials, including but not limited to metal, wood, canvas, leather, hard plastic, soft plastic (e.g., plastic bags) or expanded polystyrene foam (e.g., Styrofoam);
6) is encased in an outer shipping container covered in shrink wrap or stretch wrap;
7) is encased in a soft-sided pack (e.g., courier packs, poly bags and bubble mailers)
that exceeds 18 inches along its longest side or 13 inches along its second-longest side or 5 inches in height;
😎 is cylindrical, including (without limitation) mailing tubes, cans, buckets, barrels, drums or pails;
9) is bound with metal, plastic or cloth banding, or has wheels, casters, handles, or
straps (including packages where the outer surface area is loosely wrapped, or
where the contents protrude outside the surface area); or
10) could become entangled in or cause damage to other packages or the FedEx sortation system.
────────────────────────────────────────────────────────
but that AH surcharge would appear as a distinct Additional Handling surcharges on the FedEx info on the eBay invoice, not rolled into the billable weight. (at least it did the last time I had a FedEx shipment that had and AH SC which was a while ago)
uKfan4sure! either really had a 66 pound dimensional weight, or it is a FedEx "thumb on the scale" error.
Error can be either a literal thumb on the scale (box of books on top), or FedEx mismeasured dimensions that result in a 66 lb dimensional weight - either of which would appear as a 66lb billed weight on the eBay invoice.
That can be put to bed by knowing what the dimensions really were, what dims were used when the label was created, and what dims FedEx measured (found in FedEx website tracking info under "shipment facts" these days)
If that shows it was a FedEx thumb on scale error, ukfan4sure! will have real, true FACTS to browbeat convince eBay CS with, and use to get an invoice credit. (if that fails get Trinton involved)
To all:
Starting 1/7/2019 when the new FedEx rates go into affect there is a change in the additional handling surcharge. It is currently $12 for all the criteria. In 2019 for the surcharge amount will depend on the reason:
$13.50 per package (dimension) $20 per package (weight) $12 per package (packaging)
(reasonable to assume if a package hits more than one criterion it will incur the highest surcharge)
Be aware that neither the eBay listing flow, the calculators, nor the classic label flow have any provision for, or handle any the packaging criteria (4-9) at all. It is up to you to know that your shrink wrapped, tubular, or Rubbermaid tubbed shipment falls afoul of that, to add a handling fee to the listings to cover it as needed, and know that you will be billed an additional $12 later (and not rant about it here as fraud. 
)
*no clue if that will be built into the new label flow once (if ever) FedEx shipping is integrated there, but I doubt it. If an issue, use an account at FedEx.com.
