I ordered a pickup from DHL Express delivery and paid through a 3rd party, Paypal. I canceled the pickup with DHL soon after, before the money from my bank account had even a chance to leave. This 3rd party, Paypal, gives the money to the merchant quickly. Now that I canceled the DHL pickup, Paypal agreed to refund ME also even though the money never left my bank account to go to PayPal. I assume DHL is reimbursing Paypal. But do I need to be concerned that perhaps Paypal is suffering an unfair loss by refunding me money that I never paid?

Sure. If you know the refund is not justified, you should let PayPal know about it. If they still decide to give it you for whatever reason, that's fine. (Based on Shulchan Aruch Choshen Mishpat 348)