The realities. But hey, the Dear Rich Staff wasn't born yesterday (unless yesterday was, like 100 years ago). We understand the realities and we respect your willingness to pay a reasonable license fee, if asked. We should mention a few things that may inform your decision. We haven't seen the two works but as we have mentioned before, when it comes to portraying elements from nature, the more that the work resembles the actual thing, the less protection it will have.
No pain, no gain. As for your claim that the artist is not suffering any material loss .... We hope this doesn't sound too naïve, but if someone takes your stuff and sells it on t-shirts, isn't that a material loss? (y'know, just sayin'). Reverse roles and we're sure you'll feel the pain.
If the artist comes after you. Should the artist chase you, you do have an email trail indicating that you sought rights (which is good) but when rebuffed you went ahead without permission (not so good, as it indicates willfulness -- a negative quality when assessing damages). (A fair use defense will obviously not work for you as you are not transforming the work and you're using it for a straight-forward commercial purpose.) We can't say how you would fare in a dispute like this but we're a little dismayed that the rules of big pharma patent infringement are being applied against $20 Zazzle artists. Oh well, we'd like to stay and rant but we've got to get ready for a one o'clock presentation.