Yup, that’s what I thought. So I did the thing you suggested, I questioned it. And, as I said, the answer came back from Tesla last week: “Looking at the most up to date invoice (ie the one with the price £90 higher than my original, unmodified order), this is the same as if you were to build a new Model 3 online- therefore being the correct price”
I’m really struggling to see how that can be interpreted as “the deposit secures the price” but perhaps I’m missing something?