Although you might have already transferred your AMEX points to Delta miles during the 50% bonus promo, if you missed out or now realize that you’d like to book a trip using Delta miles, you still have the opportunity to earn some bonus miles with the transfer. For single transfers up to 99,000 points you will earn 25% bonus miles and for transfers 100,000 points and more you will earn a 40% bonus. You MUST register here first prior to transferring your miles from AMEX.
With any other AMEX transfer to Delta, you will still be charged the taxes and fees of $0.0006/mile (up to $99). Additionally, the terms and conditions state to allow up to 4 weeks after points transfer to receive mileage bonus, however, with the 50% bonus offer that just ended it was reported (and I have a close friend who can vouch for this) that bonus miles transferred over the same day. You can always put a mileage ticket on hold for two days until the transferred miles hit your account and then purchase the ticket.