What I would do is "nicely" explain to them the
situation.  SHow them a photocopy of the money
order + give them teh transaction ID of Paypal. 
Hopefully, you wrote the cel ID on the money order
and Paypal.
If they are a good dealer and have had your
business several times before, then they should be
able to work something out.  If worst comes to
worst, the last resort (if you paid through Paypal
using a CC) is to file a dispute with yoru CC company.

Hope things work out okay for you.