When you are sure you want to use a bus in Paris, France, you can buy a ticket in advance, they use the same tickets as the Metro.
As I will only be in Paris for this one cross city connection and not likely to return in quite a while and I prefer to walk the distance, I would like to avoid buying that ticket as I expect not to need it.
But if you are in Paris and caught by unexpected rain, can you hop on a bus and pay with cash or cards (credit, debit or contactless)?
(This will also be helpful to know if you still hold an old ticket which turns out not to work after entering the bus, and I have a bad record with metro tickets in the metro.)