Looks like you got your answer and that one hurts a little with the massive jump in price. In my experience the safest way is to take the box to a FedEx Office and ship it at the counter. Have them measure and weigh it, confirm the packaging type, and pay in store. You’ll get a receipt showing how it was classified, which is your proof if FedEx ever tries to re-rate it later. Most big surprise charges happen when labels are created online and the box is later audited. Counter verified shipments are far less likely to be reclassified. In fact I’ve never had one reclassified by doing it this way. Hopefully this will be the last time you have to pay an up charge for shipping! More on reddit.com

did you ship in a big box? how about a really really long box? you might have been charged dim weight L x W x H/139 vs 13lb, whichever is greater I don't know the over size limit for fedex but it might have hit that too Quite a few people seem to think this is a FedEx thing. This is incorrect, USPS, ups, FedEx will all audit postage and will claw back incorrect postage. More on reddit.com

I just went to FedEx.com and rated from Los Angeles to New York. The only rate that came close to $16 was a FedEx medium box using one rate pricing. So I guess it depends on what you mean by "across the entire US". XL box was $31.36 using standard rate. The dimensions you listed would round to 12x11x11. All non-fedex packaging gets dim weight. FedEx packaging may or may not. That's why you can sometimes see a large variances for a similar sized box. What would happen if you select FedEx box but use your own packaging? You might get away with it or you might not. Which reminds me, I had a customer complain that when she dropped off at the location where I worked she got billed one charge. When she dropped at another location, she's got charged less and wanted to know why we were over charging her. We showed her how we rated her shipment and figured out the other location wasn't doing the dim weight and was under charging her. We told her we would correct the other location. 😁 Marking residential helps you get a more accurate rate upfront among other things. Also Express saver rates didn't disappear when I rated the FedEx boxes. More on reddit.com

It starts at $30.95 for the XL. If it was 0-150 miles and the express saver service, that's the cost. The website is very clear on the brackets. More on reddit.com