Like my title suggest is it allowed to contact someone who bought my stuff trough invoice and ask him to send me a link to his video to use as my portfolio?
With respect to privacy, you should probably avoid that. The invoice data is mainly there for tax and/or legal reasons, and presumably most buyers would like to stay anonymous if given the opportunity.
I doubt anyone is going to punish you for mailing people, but I would be careful reaching out to someone who didn’t ask to be contacted in the first place. It’s a bit like if you go to the supermarket and buy some cheese, and later in the week the store manager calls you up and asks you if you could tell him how you ate the cheese. Even if some would approve of such methods, to me it’s just kind of… cheesy