sounds like computerese, I meant the title or the heading.

"voted voting slip" sounds ugly. "completed voting slip" would be much better. If your interrogation points are meant to suggest discomfort/puzzlement over this use of "slip", a "slip of paper" would be a "smallish" piece of paper (but bigger than a "scrap" of paper), roughly about the size of the receipt that you might receive when using a credit card. I think that I have herd this kind of receipt referred to as a "payment slip" on occasion.

The original in English would be more along the lines of "Please include your registered vote in the official postmarked envelope."

Sally said:

The original in English would be more along the lines of "Please include your registered vote in the official postmarked envelope."

I see, Sally. What I am wondering is how they came up with this awful translation.

Sam, I was just wondering if that was the word you would use here.

Heidita said:

The original in English would be more along the lines of "Please include your registered vote in the official postmarked envelope." Sam, I was just wondering if that was the word you would use here.
When the context is voting, the form is almost always referred to as a "ballot" (irrespective of size/shape).

We have another impossible translation on this forum:

comment back = comentar de vuelta (sic)

We'd have to know what state the reporter was voting in to even hope to figure out the original English on the ballot.

"Voted Ballot" is used in English, here for example. I would call it legalese (in comparison with your macarrónico).