RP Games is a small company, so this doesn't happen often. As per their own post, here's their open call for submissions. For those not familiar with the terminology, a company like that usually has work done by assigned people, like me, but they find those people by doing an open call for submissions. Start by emailing the address and you'll get more specifics on what they're looking for. The rate is fair. Not great, not bad. I can't say how it compares with my rate, but it's fair. Here's the post as it's appeared so far.

Call for Submissions

Rossi Publishing Games is a micro-publishing company that specializes in table top roleplaying games and we're looking to expand our line up. I can't do it alone.

We pay a base 5 cents a word and payment is made on publication (a long wait is possible).

Do you have experience creating spells, magic items, and game mechanics for a tabletop role playing game? Do you have a good sense of humor and an overactive imagination? Have you actually read this far because you're interested in submitting game data?

Send a short sample (under 500 words) of a spell or magic item for a d20 based game and tell me why I should care. carl(a)RossiPublishingGames.com

Please take a moment to look over our website or our Classes Guide on Paizo's website.

I'll also add the URL to what he's talking about here.

What happens is he takes a look at your submission (you'll get an email explaining this provided he hasn't changed how he worked it for me) and then when he needs something he matches a few people up with what they sent and asks them to submit something. He'll look over those submissions and select the one that works best. Kapeesh? Good luck.