Not sure if this is the correct forum but stick with it.
Ordered two return tickets from London to the South coast for
a four-day period over the Christmas holidays.
Went to my local station today and decided to pick them up.
.....18 tickets later - is this usual?
In future, consider e-tickets, where this is an option (you don't need to print e-tickets, but if you want a backup, you can do! Also, if there are two of you travelling, you can each have a copy of both tickets, which serves as a backup).
Also, it seems you are confused about what you bought:
Purchased two returns from East Croydon to Shoreham.
Therefore, you may want to consider using the forum's ticketing site in future. Before you commit, there is the option to view alternative ticket options, so you can see how much extra it would be for semi-flexible return tickets, rather than the Advance* Single tickets you actually bought.
We also offer an "open return" option, if you want to ensure you get returns.
(* Yes, I know not all were Advance tickets, but just having one in the itinerary means your travel times are heavily restricted).
Also, the forum's ticketing site doesn't charge a booking fee, so the most you'll be charged is the same as the TOCs charge. But if you use Trainline, it is possible that their booking fee will result in you paying more than a TOC would charge.