I've found AR-N (Enter/Print Customer Deposits) which seems to do about what we'd want. That is, to automatically apply it to the next invoice. Is this really the case and are there any other gotchas to watch out for if we use this method?

Is there really no way of putting a credit of some sort onto a customer such that it's automatically pulled up and applied at the next available moment (such as, when they order something?) It seems like this would just make sense any way you cut it. If a customer has a credit it should be applied at the first availability shouldn't it?

The gothca if you use Customer Deposits is that if they never order anything you will have this outstanding liability on your books.  If the company should go out of business or delcare bankrupcy it could be an issue since you are charging for a catalog which the IRS could look at and say hmmmm this is income.  
 
I would consult the person who does the company tax returns for clearification.  
 
We do technical manuals for one product line and would send them to anyone who requested in order to get them interested.  That got to expensive so we put it on our website in a printer friendly and downloadable file and then send the technical manuals to those who actually make purchases.  We also give customers price breaks if they purchase a certain amount.
 
We also at one time did redemable vouchers that were numbered.  When they were actually turned in we recorded it as a discount with its own seperate discount account code to keep track of to see if it was worht it.  It didn't take off.  Good thing it was a pain to reconcile the numbered vouchers with the list of redemed ones.  And if you don't number them well with today's color copiers who is to say they just won't make a bunch of copies and turn one in every time they order.  
 
You say small fee - just a thought, did they take into consideration how much time your A/R person is going to be spending to process entries/payments for one off catalog sales plus the time the sales manager will spend going thru each of these one off's to follow up on sales leads (if you do that kind of thing).  Also are you planning on only taking credit card payments?  There are fees for that too.  If not the A/R person will be even busier making collection calls.  No that would be a pain!  
 
If you can't tell - As the person who does the books - I would never recomend it to much work.