I'd make it cheaper, lower the chances of delay, and make it stackable 
The thing is, when I was trying to make it stack well, the individual cards became quite useless, as the % was so low, and then only useful as stacked, which forces you to have more than 1 in a deck, which would be a shame....
tricky one :s examples of % and stackability welcome!
In general, thanks for feedback!
