I can’t seem to find those invoices I had copied for you. Or do I just say -"that I copied for?

The first one is missing a 'that' > that I had copied for you.

If you use the past perfect (that I had), then you are talking about an action that was completed further back in the past. However, you use 'had' when you have a more specific time frame. e.g. 'I can't find the copies that I had copied for you forty minutes ago.'

If you use past tense (that I copied), then the implication is that the 'copying' part happened in the past in general, and the time frame is not specific.

So, depending on your level of specificity, either one can be used correctly.

(I think)