Anyone who uses a tiny envelope as an SASE for a normal-sized query letter already has a mark against them when I open up their query. It's not fair, but I'm declaring it. I hate stationary envelopes. Hate them, hate them, hate them. Anyway...
Did you sell the series as as series? Or did you sell the first one as a standalone, with hopes that it would sell well enough to get the subsequent volumes picked up as well? I keep reading conflicting advice about whether to tell an agent that your book is the first of a series or not.
I sold it as a stand-alone first book even though both editor and agent knew very well I had a series that was mostly written. Remember that this was a case where the editor made the offer first (she took unsolicited submissions) and then I got myself an agent. She discussed (briefly) a multi-book deal but dropped the subject when I said I had an agent, knowing that the agent probably wouldn't go for it.
First of all, once you have an agent, tell them everything. Well, not what you had for breakfast, or what you think of their new skirt, but the point is, holding back from your agent will only hurt you, not help you.
Second, multi-book deals are a complicated thing and they're pretty rare except in certain genres, or if the author is already fairly established. That's probably why I've never personally seen the contract for one. Also, there has to be serious money on the table for a multi-book deal to happen, because if the first book takes off, you've just lost possible high advances for the rest of the books. It also doesn't guarantee that the publishing house won't drag its heels on publishing the books, especially if the first one doesn't do well, or break the contract altogether.
Further complicating it is if it's a series or not. Some well-known authors (well-known in that they sell a lot of books) get deals where they promise to write a few books in a certain genre for a certain amount of money over a certain period of time. The books are usually only connected by genre. Then there's books that are actually a series, meaning one follows the other, with the ultimate intention (most of the time) of ending. Publishers like series a lot because each time a new book comes out, there's be some buying up by new fans of the older books, and they make a considerable portion of their money from their backlist. On the other hand, they're slow to commit to the first one, especially if it doesn't stand-alone (meaning, it requires the other books for the series to end, so they have to buy the whole trilogy and whatnot). They're trying to forecast the market and not lose money, like any company that wants to stay in business, but any browsing through the sci-fi/fantasy section will tell you that publishers love series for that genre. (Romance and thriller, not so much. Certainly not thriller, but sometimes you can get a "series" of romances that are different characters but the same setting)
backlist - earlier books that are still in print because they're still selling. Backlist books are more likely to have earned back their advance and are now just making more money each printing, as all of the work (except for printing the books and shipping them) is done.
frontlist - books currently being published and promoted by the company.