Welcome to our Community, we are sorry that you're struggling to add some channels to your package; this should be a very simple change that you can do via your online account or even via your Set Top Box using your remote control 🙂
Premier Sports is now £12.99 a month and comes in a bundle with BoxNation as @japitts advised.
What happens when you try and order via your Set Top Box? Do you get any errors on screen?
What happens if you try and upgrade via your online account? Do you get any error messages?