She just needs to treat the magic bags like they're cars. An expensive purchase that requires constant refills of fuel. She could sell them to major merchants and nobles for a super high price. They'd definitely buy them and because of the pricing and the low volume, it'd be worth it for her.
The recourse for fuel isn't ubiquitous as gasoline. So far, she is the only one that we know of, that can recharge this tool which is a major stopgap to scaling up.
If she sold them, the government/military would immediately demand a monopoly. It is too valuable to potentially land in the hands of rivaling nations, and not prioritize their own.
It really depends on how long the charge lasts, if the charge depletes if not in use, and how taxing it is to charge.
1) The time limit of the charge determines how far you can transport in one trip per bag, or how many trips you can make.
2) If the charge depletes over time regardless of use, then you could only use them in parallel to expand how much you could transport at the same time.
3) The tax on the charge determines the maximum number of bags that could feasibly be in circulation. Depending on the amount and duration she could be treated as a battery working full time doing only this.
Imagine if the tax wasn't so bad and it lasted about a week or more. You could sell a few as a side gig and not have it interfere with your alchemy life too much, similar to how you described.
But what if the duration was less than a day? This would require the client to interact with the alchemist once a day or more. This is a burden on the alchemist and the client that might not make it worth it for either, unless they already interact that frequently anyway. At that point it might only be practically usable for the MC and her employees. Or rather than selling a tool she could become a tool selling a service.
I suppose in you analogy if she were the only gas station in existence that sold cars. Is she really selling cars or just the service of providing gas?