Well this is my script you're using so It's relatively easy to understand;
if (rand(0,9) == 2){
getitem 26080,1; //Credit
specialeffect2 EF_TEMP_OK;
mapannounce strcharinfo(3),strcharinfo(0)+" has caught a ATM Credit Card x1!",bc_map,"0xff77ff";
}
This would be 10%
rand(0,19) would be 5% and 1% would be rand(0,99)