How/where did you buy the tickets? From DL directly or did you use an OLTA?

Try to look at the fare calculation lines on your receipt. It should show the fare class and exact fare code applicable to each flight as well as the portion of the basic fare that applies to that fare component, plus taxes and fees.

Are you sure the change fee is only $150 per ticket? Also, could you have purchased a ticket that is not only nonrefundable but also totally unchangeable (fly exactly as written or all residual value is lost)? Such fare rules are common for RT TATL tickets originating in Europe on many routes. The $89 might constitute only taxes, which tend to be refundable by law.