Minerva, I'm pretty sure that because the new system won't allow PWC to refuse the Direct Pay option at the start of the process, your DP wouldn't have the money collected by the CSA and therefore wouldn't have to pay the 20%.
C&P from my earlier post, which is my understanding of the new system from the information in the consultation document:
Parents will have to access some sort of gateway system which will advise them about making a private arrangement, offer details about mediation etc. Once they have gone through this they give the parent a unique code to then go through to the next stage.
One parent then applies to the CSA, for a one off £20 charge (obviously this will mostly be the PWC). The CSA will access the NRP's income for the previous year via HMRC records, make an assessment of how much maintenance is due and create a schedule of payments for the next year which will be sent to both parents. They will reassess this every year for no additional charge.
The NRP then pays the PWC directly, either by a method decided between them or, if preferred, the CSA will direct them to (or possibly provide, it wasn't clear) a go-between service - something like Paypal I should imagine - meaning that they don't have each other's bank details.
If the NRP pays on schedule, the CSA's involvement stops there, save for reassessments.
If the NRP doesn't pay, the PWC would contact the CSA who would (supposedly) move to having the NRP pay the CSA and the CSA pay the PWC. At this point, collection charges of 20% to the NRP and 7% to the PWC will be levied. So if the maintenance should be £100, the NRP will pay £120 and the PWC will receive £93.
If the NRP then starts to pay regularly, s/he can request to go back to direct pay. If the CSA consider that s/he will continue to make payments via direct pay they will ask the PWC. The 7% charge is meant to be a deterrent to the PWC saying no at this point.
If s/he still doesn't pay, the CSA will move to DOE, court action etc, and the NRP will be given a fixed rate charge for those (£50 for DOE for example).