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27x18x18 is too big; 26x18x18 is 62 linear inches (and 8,424 cubic inches). It's common to see 20x20x20 boxes on international flights, although if you could find a 22x20x20 box somewhere, that's probably the closest you can realistically get to the maximum volume (8800 cubic inches).

Unless you're carrying a lot of bulky, light objects, I'd be at least as concerned about the 50 pound maximum weight. If your bag is oversized and overweight, you will be charged $400 -- the oversize and overweight fees stack.

Don't expect any leeway at all on weight -- the scale is right in front of them. As far as linear size -- I agree with the previous poster. You may get leeway when traveling with bags that are shaped like luggage and don't look outrageously large, but for boxes and other less common items, expect them to grab the measuring tape.

I actually sent back an extra-large suitcase that I ordered once after discovering that it was about 6 linear inches beyond UA's maxima; instead, I had to run out and buy two smaller suitcases the morning of our trip. I wasn't willing to take the risk, and considering the high cost / inconvenience factor if UA had fought it, I think I made the right choice.

Have you already purchased your ticket? Do you have any leeway in your departure? If you can fly from San Antonio instead of Corpus Christi, you can likely find a flight that has a first-class cabin on the short SAT-IAH flight. The upgrade cost on this short of a flight is usually low -- often $100 or less -- and you'd get an increased luggage allowance.

Otherwise, I'd consider shipping boxes separately, especially if you have anything that's not time-sensitive. A surface trip to the UK will take a lot longer than your flight but likely cost a lot less than $200 per box.

Good luck!