>>покупи" (russian) -- this is incorrect. There is no such word in Russian. Kypi is a word, but I doubt it's used in the forms. May be the whole 'Purchase' example is not very good, as I don't remember having button saying Purchase in English forms either.
>>
>>I am thinking we will use present indefinite form in Russian, so it will be 'Kupit' '
>>
>>"покупитъ". -- no such word in Russian, correct one is
>>
>>"купитъ".
>
>I guessed from "покупателъ", which may be a word I imagined...

This is a correct word.

my Russian is best left read-only.

I don't think Russian is a very logical language, as that word exists, but not the other one. I am wondering if the root is 'kup' ? I am not 100% certain here.

But I figured if I made a mistake, some of the resident speakers would jump in :).