Invite Letters in AFK Journey (AFK2) are used to draw Heroes from the Mystical House. Read ahead to see a list of ways you can acquire Invite Letters, both for free and via purchase!
How to Get Invite Letters
Obtain from Journey's Gift Event

There's an ongoing Event in game called Journey's Gift that awards players with 10 Invite Letters for every day that they login to the game! The Journey's Gift event will go on for a total of 7 days, with 6 days awarding Invite Letters, and the 7th day awarding an Epic Hero of your choice.
This event has no expiration and can be claimed as soon as players progress far enough into the story to unlock the Events function!
Acquire from Treasures

Some Treasures and Crimson Treasures may contain Invite Letters once you've unlocked them! Make sure to locate and find as many treasures as you can along your (AFK) journey!
Complete Growth Path Objectives

You can acquire Invite Letters by clearing Growth Stage challenges. Head over to the Growth Path menu by heading into the Quests menu and selecting the Growth Path tab.
Engage in Honor Duels

Honor Duels are a form of PvP in AFK Journey that involves drafting your Heroes alongside your opponent. By winning 15 duels, you can acquire 2 Invite Letters a week. Sure, it doesn't seem like much at first, but the amount starts to compound after playing the game for a little while.
Receive from AFK Progress

Progressing through the AFK Stages and reaching certain Milestones in the AFK Progress will award you with Invite Letters! Make sure that your team is always as strong as possible by increasing your Hero levels and equipment to breeze through the different AFK Stages.
Purchase with Diamonds
Diamonds are one of the game's premium currencies. As such, it is possible to purchase Invite Letters by spending 300 Diamonds per letter! So save up those diamonds, and pull from the banners that you're interested in!
The game gives out a lot of free diamonds, and you don't always have to spend irl currency just to obtain them! So even though you need to spend Diamonds, that doesn't necessarily mean that you have to spend your money.
Purchase from the Emporium
The Emporium is a collection of different stores within the game. Here, you can purchase a variety of goods like Invite Letters by spending different in-game resources.
Below, we'll list out which stores in the Emporium offer Invite Letters for sale!
Recruitment Store

Invite Letters can be purchased from the Recruitment Store for 280 Dolly Tickets each. These Dolly Tickets are acquired primarily through the Noble Tavern, as each Hero recruited awards the player with 5 – 10 Dolly Tickets.
Guild Store

The Guild Store is an alternative method where players can spend Diamonds daily. Purchasing tickets through the Guild Store is cheaper than spending Diamonds directly on the banners, but the caveat is that you can only purchase a limited amount of Invite Tickets per day and per month.
Conclusion
In my experience, acquiring Invite Letters almost feels like a passive activity, as playing through the game and exploring the map typically results in accruing a handful of these ‘summoning' tickets.
Simply play the game and progress through it at your own pace, and you'll be surprised to find that you've already got enough tickets to attempt pulling for your desired Hero!
If you're looking for more guides on how to engage in Hero Summoning, make sure to check out our How to Get Hymn Conches, How to Get Echo Conches, and How to Farm Stellar Jades guides!