How much did you pay for the priority service, and what was it called on the site?
I have only ever heard of it being offered to purchase on the VFS website - it's never even been an option to purchase on the application site in the past. The old application site (visa4UK) was for applications for all countries, and each country has a different method of submitting the application, and different priority services, so you would have to go to the VFS site for your country in order to see what priority services were available to you.