Perhaps it will be based on the current price on your transaction? Similar to how we purchase on exchanges, so don't make things complicated on your end.

Definitely, there is a possibility that those vending machines are going to accept bitcoin thru lightning network, after all this is why we have LN on the first place, side-chain so that transactions can be processed quickly. LN has been tested in EU already so I will not be surprised that vending machine companies will be one done targeted crypto niche.