Hatcheries ship two different kind of "packing peanuts". One is if the weather is cold and they think the chicks need extra chicks with them to stay warm, they will put in extra chicks. These are likely to be roosters and will probably not be of the same breed you ordered. It is a good way for them to get rid of extra roosters that they would just kill otherwise. Some people don't like it but I don't think it is all that bad a practice.
The other type of "packing peanut" is that they will often stick in an extra one or two in case one dies. It's easier for them to stick in an extra chick that to process a claim for one or two dead chicks. These are usually the same thing you ordered.
For example, in May of last year I ordered 27 chicks of four different breeds from Cackle and they shipped me 28 chicks total. The extra was a Buff Orpington. I had ordered 6 straight run Buff Orps and received 7. All 7 were female although I ordered straight run. The odds of that happening was less than 1%. So I am convinced the extra Buff Orp was just grabbed out of the straigth run bin and added to my order in case one died. All 28 lived.
I cannot tell you what Cackle will do in your case, just what they did in mine. Good luck!