I would be careful with having too much money on those. I find them really prickly. I get them for doing surveys and things online and I have had a difficult time using them. Disney would not take them to pay down my trip. So many other online sites wouldn't take them, I couldn't pay bills online with them. I finally found one bill online that would take it. This was a Visa card where I had the #, exp date, code on back and it was associated with my name and address.
I'm not saying they don't work and if you have only have a few not really a worry. But, I wouldn't trust them for my trip.