I certainly understand the fear. I've trimmed a little too close before and drawn a little blood and it broke my heart to make my dog bleed. I have a small chihuahua who hates having her nails trimmed. I started with a normal pair of fingernail clippers, I figured since she was so small they'd be fine. However, I discovered when I used larger trimmers (like the ones you're probably talking about from PetCo) they seemed to not put pressure on her nail as much while I was trimming. Consequently, she tolerates her nail trimming much better now. I also use the grinder on her nails afterwards to smooth off the sharp edges, and she doesn't seem to mind that part of it much at all.
As an extra tip, keep a bar of soap handy while you're trimming. If you do trim too far, you can rub the end of the nail on the soap which will stop the bleeding. This doesn't hurt the dog either. Good luck!
