Re: slides without background...
I can't help with your silver taping question but I'm pretty sure it will be cheaper to shoot the extra slides yourself rather than get copies made.
My only guess about the silver taping would be that you might want a neutral gray background instead of one that is either tinted, too light or too dark. Maybe the tape is one way to achieve that? I would think that a can of paint would be a better solution. I bought a quart of 18% gray paint at a local home improvement store for a few dollars.
I hope this helps.
I welcome comments and suggestions on the composition and subject of my photos. Suggested crops and color, sharpening contrast adjustments etc are also welcome. Manipulations that alter the pictorial content are right out.