This isn't exactly a report of a scam, but more of a "PAY ATTENTION" warning.
While looking for the appropriate site to apply for an ETA for a trip to Scotland, I had to scroll by several sites that offer to "help", in other words, charge 5 or 10 times as much for the same service.
Make sure you go to the official UK site. I happened to do it without the app, and it took a very short time. The only issue I had was that there was no way to avoid DCC - I was told I had to pay in dollars.
My apologies if this has already been discussed to death...