@sensesis I'm glad you found it helpful!
It always seems that "budgets are tight" doesn't it? ... sigh... 
I didn't realize that it was the agency commissioning you. Their contract with the client (if they have one) may provide additional parameters, but aside from that, I'm assuming that your contract is directly with the agency. Your parameters should carry through to the end client; however, the agency may not convey those terms to the end client...
Regardless, it is good that the usage has been spoken of ("for a three month period" "on the box and the instruction sheet and that additional usage would bear additional costs"). The ideal is for this to be formally written and mutually agreed upon in a clear manner (as opposed to a verbal discussion).
When I give estimates, I like to detail out what it covers and what it does not cover in a very clear & thorough manner.
This was extremely necessary with web development projects, because I didn't want the client to assume that I was paying for hosting costs, or developing and/or paying for stock imagery costs, or writing the content (or paying for a writer), or paying for additional functional development, or developing more stuff just because they thought of more things they wanted to add deep into the project, etc. etc. etc.
It's all about each party managing expectations.
Currently, by the way you described it, you are probably somewhat covered by the literal imagery you are producing only to be used for 3 months on the box/booklet and for no other use; however, as I understand it, there is nothing stopping them from hiring some other illustrator to redraw the characters in other poses for continued use. Maybe there's a case against this, but I'm not a lawyer so I don't know. The best is to have it explicitly stated in the contract. Then you have something to go back to should there be a disagreement of work expected, usage of work/elements, etc.
Per my view, the short answer would be: No, you probably cannot technically charge for character design in future illustrations without this having been agreed upon by both parties prior to the development of those characters. Currently, you'd be creating characters for the purpose of making the best illustration possible.
BUT perhaps if you were the illustrator to do the next illustration work, you could then find a way to recoup these costs through some sort of line item(s) like "extended character development" or "character standards development", where you convince the agency/client that this is necessary for character continuity/consistency with ongoing projects due to their desire to keep using the specific character(s). Of course, this could get a little tricky with how you approach, convey, and negotiate this. If they don't have money now, chances are they won't have money then either.
BTW, unless you have agreed to this, I would not hand overâin fact, I would not even showâorthos (front, side, back, angle) views of any characters nor facial expression exploration nor multiple pose exploration (other than that which is necessary to complete this particular illustration project).
You can do those things if you want (knowing you won't be paid for them but just because you want to do them), but the moment you show them, the client will want them and then will have them for future character development, without having paid for them (it's just extra stuff for this project).
Lastly, to be clear, these are all just my 2 cents. I'm no expert by any means and could be totally wrong. I just know how difficult it is to try and balance both "doing our creative thing" with the necessity of business engagement, and I wanted to share my current perspectives in hopes that they would help or generate a discussion here to help us all.
( ^ - ^ )
