Missing the plastic bag for shipping for my amp...

Is it actually possible for styrofoam to permanently mark up the steel chassis of the amp?

I would go to a store like best buys or whoever and ask for the plastic bag that a computer was packaged in.

Yes those styrofoam end pieces can scuff up the finish but the suggestion of the Glad Wrap is your best bet. Manufacturers use bags because it is less time consuming and labor intensive.  If you're sending it back to manufacturer they will put it in a new bag as its faster for them. 

Roll of visqueen from a big box hardware store.  

ULINE sells plastic bags in several thicknesses, in a dizzying array of sizes.  No doubt, they have what you need, and may even supply the manufacturer