Lucky Voucher is an Item in Monster Hunter World (MHW).
You can use a lucky voucher when posting a quest to add bonus rewards upon its completion.
Lucky Voucher Information
Lucky Vouchers are obtained once per real-life day with the Login Bonus. In the event you miss a day, you'll still get all Lucky Vouchers in the next Login Bonus. However you can only have a maximum of 5 Lucky Voucher so be sure to use them before checking the Login Bonus.
Lucky Vouchers double the amount of Zenny you get at the end of a quest, stacking with the Investigations Zenny bonus. It also allows you to get more drops as quest rewards(guarantees receiving the max amount of quest reward boxes), upping the chances of getting rare drops slightly.
Each member of a group can use one Lucky Voucher per quest. Lucky Voucher effects are NOT shared between every party member.
You can check how many Lucky Vouchers you have by going into any quest category and looking on the right border of the screen, next to the map.
Effect on Quest Rewards
There's lot of confusing information or misbelief on the effect of lucky voucher on quest rewards. First thing is to not confuse Hunt Rewards and Quest Rewards. Both appear in the same large box of rewards but the hunt rewards are 4 to 8 rolls of monster materials for each monster killed or captured during the quest, the quest rewards are 4 to 8 rolls of rewards from the pool of actual quest rewards for the completed quest. That's why quests like Day of Ruin state only having feystones as quest rewards when they still give teostra materials in the quest rewards category when completing the quests, that's because the teostra materials are part of the hunt rewards(for killing teostra in the quest) and the feystones are part of the quest rewards(for completing the quest).
Lucky voucher will guarantee the 8 rolls of quest rewards(instead of eventual 4, 5, 6, 7 rolls of quest rewards that could have be gotten otherwise). Hunt rewards are not affected by lucky voucher. One way to realize it, is to do the quest Brand New Brute as it doesn't give hunt rewards because the game lets the player fight a Brute Tigrex before MR 70. The Lucky Voucher icon left to the Quest Rewards text and the golden glow behind the quest rewards category also shows or confirms a lucky voucher was used on the quest.