You aren't actually charged until you hit 'confirm' when the new price appears. Sounds like a good strategy, but they won't side with you on this one is my guess.
Which could be why delta.com has the following disclaimer on the Trip Summary page:
The advertised price is not an offer, and is subject to change prior to the completion of the purchase, which may take up to 3 minutes to verify seating availability at the advertised price and your credit card information, so please be patient. When you click PURCHASE, you agree to accept all Fare Rules for each flight and all terms in Delta's applicable Contract of Carriage and authorize Delta to charge your credit card. No contract exists until you receive confirmation that your credit card payment has been accepted.