Remember - it's YOUR work that they want and need. You have the right to ask for whatever YOU want to make this feel right to YOU. You should be stating YOUR terms upfront; don't wait for them to set them first and potentially walk all over you. They may object to your proposal, but you should at least launch one as a starting point for negotiation. You should also research comparable licensing fees first (i.e., for licensed photos, or whatever), so you have some data you can share as backup for your proposal.
I strongly advise against giving away work for free (or just the cost of the one cookie), as it only makes it easier for the next company to expect to get work for free from the rest of the cookie community. (Can't tell you how many times I've heard, "But no one asked me to pay for this sort of work before.") Let's establish our worth, and expect to get paid for it! Doing so will elevate the entire cookie community.
Also, think creatively about compensation. If they are a small company and don't want the hassle and cost of royalty accounting/reporting (especially on a relatively low value item), then maybe there's another ongoing benefit that you can negotiate - like everyone who buys that balloon gets your business card (a referral), or your contact information is listed on that page of the balloon book, etc.