Ok this story is now over, but let this be a WARNING to all (Relatively) new sellers using FedEx (or UPS for that matter) shipping through EBay/Paypal:
EBay does not acknowledge any errors in their shipping tool with regards to Dimensional Shipping. They do not give you any warnings, and claim they are covered by the "Estimated Shipping Cost" label they slap on the price. YOU as the seller are supposed to go to the FedEx/UPS website, enter package details and figure out what the dimensional weight is for your package, then use that weight in EBay's shipping tool to calculate your package price. THIS IS NOT EXPLAINED ANYWHERE.
EBay could have just included this calculation in their tool, it's a one-line math formula (HxWxDxfactor) using data they already collect from you, but they are not doing this.
I had 4 phonecalls over the course of 2 months on this matter, which just became a matter of principle to me, and ultimately after speaking with the manager of the supervisor of the agent I worked with initially, they gave me a coupon to cover the difference in quoted cost and charged costs, as they claimed they could in no way refund me the difference (I was quoted $13.55 for a FedEx smartpost shipment, and charged $27.38 - got a $15 EBay coupon expiring in 2 weeks).
I strongly advise everyone here to either avoid using FedEx/UPS when shipping from EBay, or to bookmark the dimensional weight calculators for use before purchasing a label through EBay. For reference, the links are:
FedEx: http://www.fedex.com/in/tools/dimweight.html
UPS: https://www.ups.com/content/us/en/resources/ship/packaging/dim_weight.html (but this only gives you the formula)
A combined tool is available at ShippingEasy.com if you are comfortable with using a third party calculator for this: http://shippingeasy.com/dimensional-weight-calculator/
If I had the same amount of balls as Tabby, I would have taken them to court over this, as I think this is a scam. The comparison I used with them is the following: "If I were a seller on EBay and I offered something up for sale for $100 with $10 shipping, and then charged the buyer $20 for shipping after learning the true cost, without their consent, my seller account would be banned immediately. Why do you do this to your customers then?". They admitted it was not correct, but could not refund me the money, nor commit to putting even a simple warning popup on the calculator to inform people that they need to check their dimensional weights before using the tool.
Now what to buy for $15 Can you use EBay coupons on EBay GCs ? *wink* EDIT: YES