Save 0779222777 as moAirtime then send the word BUY to moAirtime through WhatsApp

How do i pay for my airtime?
Through MPESA. You will be prompted to enter your PIN then the amount will be deducted automatically
Are there any hidden costs when i buy through WhatsApp?
Will i incur MPESA charges if i buy huge amounts e.g 1000/=?
Do i enter Airtel, Telkom or Safaricom number every time i want to buy?
Can i buy for another number ? If Yes, How?
Can i change the MPESA number that i pay with ?