well how they justify it is that they handle the customer support. which in turn keeps you cb rate low. plus they bundle you with tons of other people to also help keep cb ratios cool if a few people cross the threshold. of course they do all this groovy scrubbing (over scrubbing) to help with cb issues too.
all in all, they provide a nice service but i think 10-15% is too high. i would be happy to see all the services provided by a ccbill or a paycom at 6-8% with no volume commitment...
