O2 upgrade

I have a Samsung S6 and the battery keeps dying. Luckily it's time to upgrade. I did the online chat and accepted an offer which was for an S7. I have to pay £29.99 upfront then with a discount the monthly dd will be £24 instead of £29.

I was happy with this but it took forever to complete. I was then asked to click on the link provided and it would take me to the order page. On here I had to provide my bank details and agree to pay £29.99 up front and £29.99 a month. I queried this and the guy on the chat line said that I have to pay the £29 up front, receive the phone, then when the tariff starts, phone and ask them to add the bolt on.

I refused to do this. Is this normal practise? You wouldn't go to Tesco, bill comes to £24 but you have to pay £29 and then phone them up when you get home to ask for the refund!

How do I know the bolt on will be added. There was nothing mentioned on the order page.

Should I still go ahead with it or not?

Thanks