I don't think another site has ever stressed me so much trying to buy something.. Where to start..
Firstly, it's worth mentioning I was on the mobile site.
So I add 500ml of nicotine to cart and continue browsing, when I decide to view cart I find the cart is nowhere to be found.. I think "I'll add some more nicotine to cart because you get a menu temporarily appear on top of the screen when you do that" only to find no option to remove things from cart either.
So now I'm stuck with a litre of nicotine in cart and not an option to even view it.
Then I think "ahh, I registered the last time I used it for nicotine.. Hmm I'm no longer registered and no option to re-register.
I nearly went somewhere else to buy it in the end but after a little screaming into a pillow (I'm joking) I distressed a little bit and wrote this instead.
Edit: I succeeded in the end by using a different browser which had an empty cart, adding nic to cart then clicking the cart without leaving the page (because cart vanishes if you do) and going straight through checkout.
I know you've recently updated your site and you're still ironing out problems so please see this as feedback..