I don’t have experience personally with such deals, but I do have long business experience overall…and my business instincts tell me that you are selling yourself short. Especially since you put in extra work to keep them every week. You have basically made yourself very hard to exchange, as the entiresworkflow is dependant on you. That is a great position to be in for any negotiation!
I can’t really give you a number, but since they want unlimited usage, that should be reflected. I assume this means they also want exclusive rights to the track…well then…I would personally not do less than 10x for unlimited exclusive usage.
Remember, for a chain with 172 supermarkets, this is pocket change money for them anyway. And if they have been happy using your song, to the degree they even ask for this unlimited use, well…I think you have a good chance to get a substantial increase.
Good luck, and let us know how it goes!