It's great when you get a sale from a magazine. There might be a note in the mail (sometimes in the same SASE you sent to them), you might get an email, you might get a phone call. I'm putting these modes of communication in descending order of probability. Most sales I've gotten I've either learned about via regular mail or email. I receive more email in the past 5-6 years, now that I'm in more editors' Rolodexes.
So, those silly little drawings with the captions under them that you sweated over and mailed out -- one of them sold!
OK, so, congratulations! Now what?
Now, you have to get your cartoon to them and get paid.
Sometimes, the magazine may have all ready pulled a copy of the cartoon from your batch and they will print from that. Other times, you'll get it back with a note that they want to buy it. Sometimes, they'll ask you to redraw it. For instance, the Wall Street Journal always wants my cartoon to be drawn square and in a bold line -- which tells you right there I'm a wimpy line/horizontal kinda guy.
Most magazines do not have size restrictions. Only one (Playboy) tells their clients how many picas high, how many picas wide a cartoon should be. I remembered "picas" from journalism class. I had to call the cartoon editor and ask for a translation into inches. It was no problem. (Most of the time when I call an editor with a question about a sale, they're happy to help.)
Anyway, with most mags, like the old wheeze goes: size doesn't matter. More than half of the editors have little or no opinion about whether you should just sell the cartoon that they say, or draw a "finish" of that same cartoon.
Back in the day -- the 1950s & 60s, when cartoonists would go in person, from magazine to magazine, every Wednesday, doing the "rounds" -- he would drop off roughs at the mags. If one of them sold, he would draw a finish.
I don't mail out roughs. What you see is what you get. There is more than one mag that tends to go through cartoon editors every 18 months, so I always try to do my best job, thinking that there may be someone who is looking at these cartoons and they do not know who I am. So, no faking a sketchy looking drawing.
If the editor has provided contact information, then I contact the editor (I usually make a phone call, especially if I don't know the editor) to (a) say thanks for the buy and (b) ask if I can email the cartoon. I usually email a 300dpi JPEG.
Most publications will want you to invoice them. An invoice is a bill and it doesn't matter what it looks like, so long as it has information I tell you about in my Cartoon Invoices blog entry from June!