No, Senti Pay is 100% free for all users to send and pay cryptocurrency globally at any time. The only fees a user will pay are miner fees from the blockchain - however the vision behind Senti Pay is that it supports coins with very minimal miner fees of less than  $0.01 at most.

No, Senti Pay is a non-custodial wallet therefore we do not hold any private keys of any users. Thus, all users must protect and backup their private keys or mnemonic phrases to protect their funds at all times. 

A transaction could be unsuccessful because the wallet you are making a payment from is not activated or the amount is too small and is a dust value (The miner fees cannot be paid). As Senti Pay is a non-custodial cryptocurrency wallet each user has ownership of their wallet on the blockchain only, therefore to use your wallet you must ensure it is activated. A wallet is activated when it has a minimum holding balance of cryptocurrency as required by its blockchain protocol (example: the Ripple protocol requires a minimum balance of 20 XRP to be activated). We encourage users to make payments of more than $2 to ensure miner fees for rewards points, the actual payment and transaction fees are covered.

We encourage all users to accumulate a rewards points balance of at least $2 before they make a payment with their rewards points as this should cover miner fees and any additional fees required by the blockchain (miner fees do not relate only to the actual purchase transaction but also towards transaction fees being paid the merchant on the transaction.)

The available balance of the coin being displayed on the home screen will be the coin that you will be making a payment in when you tap the "PAY" button at a merchant. However when sending cryptocurrency using the "SEND" button a list of coins will pop up for users to choose the coin they would like to send.