You wanted to excess the RTN portion of a Cardiff to Aber return ticket to Fairwater? (ie, Aber to Cardiff RTN to become Aber to Fairwater RTN )
I can't see any problem with that; it's a straightforward overdistance excess, costing £0.00
Some staff claim that once you reach your final destination it cannot be excessed. While I can't find that documented, it is difficult to argue against but providing you asked before reaching Cardiff I cannot see why they would refuse.
However some staff do not like issuing zero excesses (do a search on this forum for previous topics) and will give any excuse not to do them. Most passengers will be fobbed off easily and will not go to a forum like this one and find out they were overcharged, so most of the time they get away with it. And also they will assume people won't bother about a fare like £1.40.
Sorry you experienced this. It's inexcusable but all too common, and it honestly does not surprise me at all.
This is exactly what I always find when dealing with staff about excess tickets-it's actually quite simple-if u don't know the rules-learn then-if u do but can't be bothered to issue zero excess tickets-and there are reasons why such as time and complexity-then don't! But do not fleece the customer by issuing incorrectly priced tickets-surely as illegal as a customer trying to play the system by not getting a ticket etc
I suggest the OP writes to complain and send the tickets-time consuming but it's the only way they will learn to train their staff.