Popa,
Because who says you can’t daven a nedava on Friday afternoon after you were mekabel shabbos?
I don’t understand your question. Which of the following two statements are you questioning?
1) We don’t say nedava on Shabbos.
2) One must keep all of the halachos of Shabbos after accepting it (any time after plag).
An argument I could hear is that we only don’t allow Mincha after being m’kabel Shabbos is because you can anyhow daven two Maarivs; since doesn’t apply here, one can daven Mincha. I don’t find this mistaver, though.