Opodo complaint: Confusing website and extra charges

I booked three flights Belfast Int to Liverpool (EasyJet) for 9 March 2023 and three flights Manchester to Belfast City (Aer Lingus) for 10 March via the Opodo website. The latter posed no problem but after I purchased the former flights I noticed an additional £69.67 of fees had been added to the invoice by Opodo taking the total cost from £335.97 to £405.64.

I have argued my case repeatedly today via email but their Customer Services department say that because I omitted to log in via my Prime account number for these 3 flights I did not qualify for the £69.67 fees to be discounted. They say I did log in for the latter 3 flights thus the discount being allowed. My contention is that although that would appear to be my error (I take their word for it) there are mitigating circumstances:

1. Such is the complexity of the Opodo website I, as a new and first-time customer (I was 75 last Sunday but fully in control of my mental faculties ) was confused and mistaken.
2. At no time did the extra charges show as I booked the flights, only afterwards as a total figure – surely this cannot be allowed. Opodo have still failed to provide me with a detailed invoice for the £405.64.
3. Their charge of over £69 is very harsh on a new, first-time usercustomer.
4. Opodo have repeatedly ignored my requests for a copy of their complaints procedure.
5. Opodo have repeatedly refused to offer any leniency regarding the hefty £69.67 additional fees.

Is there anything you can do to help? I know the error was mine but for one set of flights to be awarded fees discount and the other not all due to one small slip during the booking process seems disproportionate and totally lacking in any sense of customer care.

Thomas L Place