I want to pay for Ubuntu mobile, but I only have Visa (Electron Debit card). Can I use Visa Electron to pay for ubuntu one service?
Unfortunately Visa Electron cards normally aren't real Visa cards, they're debit only cards and worse they're payment up front cards. This makes them expensive and unusual to process as you need a clearing house that supports them (Electron is to Visa what the old Solo was to Switch and meant for the really poor who don't even have enough credit to have their balance protected).
Anyway, back on track, the payment methods don't currently allow for visa electron. But you may be able to get hold of a pre-paid credit card (depending on your country) which you can use to do the transaction. They come with a fixed amount of money and it's a little more work, but you could get the job done.
The other option is to ask a friend with a real credit card to use his card instead and you give them the cash.