they always want to sell me something rather huge and heavy
Can you give us an idea of what you believe is huge and heavy? I ask this because the models that many of us might suggest (i.e. Ninebot Max), could be seen as too big and heavy (40+ lbs), despite being a relatively small e-scooter.
For a distance that short, has she considered a bike (even a folding bike)? Most buses can carry them up front, and folding bikes can be taken aboard most public transportation. Bikes are also nearly half the weight of a smaller e-scooter.
Another question is do you have rentable e-scooters/e-bikes in your city/town? That might be something to consider as well.
For an e-scooter, you’d need to make sure they are permitted on the campus, as not all will allow them.