As a measurer, I prefer to get the certificates (two are always sent to me via Everett McDowell) and then I forward one to the RD when I get paid.
I wait until I receive the cert in the mail, then I inform the RD that I have the paperwork and I send an electronic bill to them via e-mail and let them know that the cert will go in the mail as soon as I see payment.
As soon as payment is made I send the cert in the mail along with a hard copy of all my maps. I send copies of everything electronically as well when I get paid. The RD gets the materials sooner than the 3-4 day snail-mail time.
I prefer this method because it insures that I get paid. Since I keep a copy, I am always able to reproduce one if necessary. I had one race that continued to forget to pay me (fraternity at a local college), and I repeatedly had to request payment. It wasn't until I mentioned legal action that I got anywhere. I hated doing that, but we are business people afterall.