I think it’s quite important to note that OP doesn’t pick Susan up. So Susan is hardly an entitled madam clicking her fingers for a taxi every time.
At 30p a gallon where a litre is £2 (so conservative) this trip back only, twice a week, is costing £2.40. And I’m well aware that not everyone has that to spend on someone else.
So I’d go with my earlier suggestion - if you don’t want to ask Susan for petrol money, then (with Jane present) explain that times are hard and can Susan bring the chocolates for Jane instead of you, and instead of giving you petrol money.
Tbh I think it’s ridiculous of Jane to take chocolate off you twice a week for “hosting”, given your low income and the low key nature of this meet. I’d have long since told you not to bother!
I still think you just don’t like Susan and tolerate her being there to see Jane. A 10 minute extra car journey, 5 mins of which is spent with a friend is not hardship - for a friend.
It’s a absolutely legitimate to need to petrol contribution. But if Susan posted “AIBU to think my so called friend could drop me 10 mins out of her way, when it’s dark and cold and rural and she herself says not safe to walk” I bet everyone would say - how is she even a friend? (though they’d also say - you’re giving her petrol money though, right?)
All the stuff you posted about how people shouldn’t just offer, they should give money, because they should know the driver will say no… that all sounds like she did offer, and you did say no.