Topping up with commercial wine

It depends on your definition of contaminate. Commercial wine has probably been sterile filtered, and pasteurized or sulfited, thus the chances of microbial contamination are small. If you added a heavily-sulfited wine to a fermenting wine, it could cause problems, but I wouldn't describe them as contamination. But if you try to add Ripple to your Chardonnay or Cabernet, that would certainly be contamination in my book.

An English tourist entered a small tienda in Catalonia, where he purchased a bota (a small leather wine flask made of goat skin). As he started to leave the shop, he turned to the proprietor and queried, "It's ok to use water, isn't it?" The proprietor's face turned ashen. "¡No, señor!", he exclaimed. "Water will completely destroy the insides!" The customer rejoined, "Are you quite certain? I don't see how water could hurt the flask." The proprietor's face returned to its normal colour. "¡Ai! la bota. I thought that you intended to drink it".

After doing this on a a number of wines, I'd say there's no problem - in fact it's a lot better than the alternatives of adding water or leaving too much head space. Your other options are to add sterilized marbles (a bit of a pain, but workable if you're topping up a wine that's hard to match, like an ice wine), or rack into a smaller container.

Ed