SOLVED:
Try another browser. I was previously using Safari on Mac OS. Tried the transaction about 3x's before asking the client to speak to their bank.
Their bank confirmed they can see the request (even had a transaction ID) and were not blocking it. So I spoke to PayPal who also confirmed seeing the transaction but said they are not blocking it, but don't know why (for some reason) it is not coming through.
Before getting off the call, PayPal recommend I try clearing my cache or using another browser entirely. I choose the latter and with the same details (and on the 6th try) - it worked!
Hope this helps.
