Flyer, who I write for from time to time, pay on invoice. I think they take the attitude that if you don't send an invoice they assume that you don't want paying (a thoroughly reasonable attitude, prevalent in business.) They have a rate of so much per 1000 words and so much per piccy, so it is easy to calculate what the invoice should be.
And I reiterate the comments above. Send them an eMail saying what the article is to be about and they will yeah or nay in principle, reserving the right to nay if they are not happy with the final result.
It is a long while since I wrote for anyone but Flyer, so I can't speak for any others, but Ian Seager (and Nick of happy memory) are/were jolly nice chaps and are easy to approach.