The biggest irony in the story is of course that Jim has sold his watch to pay for the combs for Della's hair. But Della has sold her hair to pay for the chain for Jim's watch. So neither gift is useful and both have sacrificed the requisite thing to make them useful or meaningful in a material sense.
The sense of surprise in some ways is muted because neither Jim nor Della gives away their surprise at seeing the other without the watch or the hair, but they are both surprised at the gifts they've purchased for each other.
And the end of the story suggests that the power of love negates the material uselessness of the gifts and that both Della and Jim are wise even though they don't appear so.