I have a debit card (VISA) and they accept it. Previously Google did not accept it, only credit cards. I don't know about prepaid but I think the reason you want to use one is why they don't accept them.
This is an implementation of the line breaking algorithm used in TeX in Javascript. It would be nice to add to obtain better typographic results with justified text.
I like writing in a Jupyter notebook. You can use Markdown, write equations with Python and use a module like SymPy to transform to LaTeX, create graphs, etc.
If scientists continue tolerating the monopolies of big editorials, this situation is not going to change. It is time to embrace open access journals. Meanwhile, I use SciHub.