The only casual wedding I have been to was my aunt's second wedding. (Her husband was on his third.) They basically turned it into a family reunion because they already had done the fancy services in past weddings.
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uh-oh, nothing looks worse than slacks and sneakers. Wear jeans (or Dockers/work trousers, if you have them) if you don't have decent shoes. Return the slacks and belt.
I can't believe you dropped $70 on that shirt, though. You know they sell shirts with letter-based sizes at the same store you bought the pants and belt at, right?