You shouldn't have any problem. Just talk with the
dealer about the matter and it should be easily
resolved. To clear any of his doubts offer him/her
to send an scan of the signed receipt of the money
order and also the ID number of the paypal
transaction to make him/her easier to confirm that
the payment was sent and received twice. On the
top of that if you just ask for credit on future
purchases that would make things easier for you
and specially fo the dealer.