I have noticed that there are many sites that process on-arrival visas (ETA or whatever it's called - before you arrive). Many expatriate Srilankans get caught and pay extra as these are the ones that immediately pops up on a Google search. Several of my friends who came in got caught. This has been happening over many years.
But there's no excuse if they handed over the entire visa issuance to a company that charges extra.
FYI any SriLankan can gain entry even without any travel document at KIA. Thats a right enshrined in the constitution that many are not aware of. You need to reasonably show evidence to the immigration officer at the desk by conversing with him or by calling a relative of yours who is in SL. Proof can be provided later.
(Of course you can't fly in without a travel document, that's a different story.)