Take a look at the fare bases for your itin before and after applying the coupon. We've seen quite a few folks here who have reported receiving less than the full discount, but it was because the pre-discount ticket included a PE500 or similarly designated fare on at least one of the legs. A PD886 can't be applied to a PE500 designated fare, so it finds the next-lowest fare, which is often $10-25 more, resulting in a smaller discount than expected.

It's also possible that the CSR was correct, and this fuel surcharge business is something new. I've used PD886 and similar certs several times in the past year and have always gotten the full $-off I expected to get--as long as the pre-discount fare wasn't promotional.

But I'd wager that the CSR was wrong and your issue is with PE500 or similar fare designations, because the math doesn't add up otherwise. $432-25=$407, so you should still get the full $100 off. $307 + $25 fuel surcharge + other taxes and fees = about $430. Not $456.