One payment
There are no annual fees and no mid-term renewals: you pay £20 when you apply, and the authorisation lasts 2 years — or until your passport expires.
The amount is charged even if the application is refused, because it covers processing.
Sites that charge more
Some intermediaries add their own commission on top of the official amount. They are not authorised by the UK government and do exactly what you can do yourself.
If a site asks for noticeably more than £20, the difference is their commission.
Recognising the official site
The official address sits under the GOV.UK domain. Be careful with sites calling themselves an « official partner » or « approved agent »: the UK government does not authorise third parties to submit applications.