Pay for the courier and item by ping

Sez, no, once a Ping payment has been made it can't be altered, although the seller can refund either in full or part via the ping payment. 

We don't want to alter the amount paid for the item - just the customer would now like to pay for the shipping via ping as well

Yes, like I said, once it's been paid by Ping nobody can change the amount. If absolutely necessary, you can always start a new listing for the shipping only that can then be paid, or create a new listing for everything incl shipping then cancel the first one. Easiest though is for the buyer just to pay by internet banking as they really should have chosen a shipping method initially, if there was one.

HI Sez. You should already have your shipping costs on the auction if you have ping activated. So the buyer can pay for that at point of sale with ping.

10.5. A seller must offer clear shipping options with specific pricing on all listings offering Ping.

You can set up an auction afterwards for JUST THE SHIPPING COSTS and make it payable by ping. Copy the link to the auction and send to the buyer. It works but it's messy and extra time and effort.

There's sellers out there offering "ping" as a payment but don't have the shipping auctions in that section.They have the options next to the description to save them success fees on the shipping costs which is quite silly in my view as a ping payment only covers the item & then the buyer has to make a separate payment for shipping.Very messy !