I got bump paper vouchers for $400 (4 x $100 vouchers) 2 months back and another $400 (4 x $100 voucher) paper vouchers last week for volunteering with United flight. 1. How can I use them? Seperately as $400 or can I combine them together as$800 for an International flight? 2. Can I also use them as $200 four times? 3. Can I combine one $400 paper voucher with electronic certificates?