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

but I am curious if that is only in case with a larger discrepancy, and if it is even worth our time to add the .25 or .50 inch to the measurements The packages get measured by laser and the billed dimensions adjust if necessary. You can see any differences between your claimed dims by looking at your shipping history comparing it to your invoice. The invoice will tell you the dimensions billed and if there were differences egregious enough to change the billing. Does your shipping tool allow decimals? Some of the FedEx Ship Manager fields don't take decimals. You're supposed to round up to the nearest inch (or switch to centimeter units I guess). Would love to know if other shipping tools take decimals. I use box specified dimensions if available. Occasionally the lasers add an extra inch somewhere, usually not big enough to change anything. Good luck if you need to dispute something, refunds are basically off the table this year. More on reddit.com

Fedex office stores can custom make a box but that costs a pretty penny. Uhaul or uline stores normally carry large boxes too. More on reddit.com

From my experience, 22 inches is the max for total box dimensions. For example, a 10×6×6 3lb box is $7.99 FedexEconomy. A 10×6×7 3lb box is $10.48 Fedex Economy. I would suggest entering box dimensions in "how big is your item" for accurate prices. More on reddit.com