What's wrong here? "Once shipped, a customer will have to wait up to 14 days for the gift card to arrive in the mail."
The number should be spelled out.
The sentence is wordy.
The grammar is incorrect.
The sentence is just fine.
Congratulations to 65%! A customer--I hope!--doesn't get shipped. This is called a misplaced modifier. Here's a less humorous version of the sentence:
"Once the gift card is shipped, a customer will have to wait up to 14 days for it to arrive in the mail."