Delta Airlines stole $40 from me Atlanta, Georgia

I recently purchased two tickets to Florida via Delta.com.  While purchasing I noticed they had a Best Fare Guarantee insuring I would get the best deal.  Therefore I felt safe and purchased the tickets there.  Well, after I purchased I figured the least I could do was search around and see that they were indeed the best deal around.

When I searched, I noticed that travelocity.com and nwa.com had prices of $20 per ticket less.  So, I went back to Delta.com and I filed a Best Fare claim.  They promptly emailed me back saying they would NOT give me the credit because my flight is being handled by Northwest Airlines.

This was curious to me, as Delta now owns NWA, so I emailed them back and asked why this is?  I was told that this guarantee only applied to flights wholly operated by Delta or "Delta Connection" carriers.  Apparently, even though Delta now owns NWA, and many of NWA's partners are Delta Connection carriers, NWA themselves is not considered part of Delta.

Needless to say, I feel that Delta Airlines has not only stolen $40 (and two $100 vouchers for a future flight) from me, but also my confidence in going forward with doing business with the company.  If this is how Delta treats its customers, I'm not sure I want to do business with such a company in the future.