Buyer paid for expedited shipping – First Class USPS has same ETA as Priority

I just had an order come through for an item well under a pound with expedited shipping. I charge an additional $10 per order for expedited shipping, but offer free standard shipping. I looked up shipping options, and it suggested First Class USPS since it had the same ETA as Priority (which would cost twice as much on my end). I feel a little weird about shipping it first class since this isn’t something they needed to pay additional for. What should I do? Refund their shipping? Send it priority? Enjoy the extra cash? Thanks!