Flat cards leave you less space to write.
At first this seems like a downside right? WRONG. In many cases you want to just send someone a short and sweet note, but if you have a large card, it can feel like you either have to write super big or write a lot to fill it up. More is not always better and this is my main reason for always using flat cards. It takes the pressure of feeling like I need to write a book!
Flat cards are less expensive.
Less paper and less labor means they are the cheaper alternative. With postage costs rising each year, if you have to send loads of thank you cards, you may not want to spend that little bit extra on each card - it will add up.
Flat cards don’t need to be opened to be read.
Sounds like a silly thing to care about, but think about how often people who are touched by a thank you note want to hang it up on a fridge or wall? If it is a folded card, the shining thank you or sentiments that you sent will not be seen unless someone walks over, opens the card, then stands there holding it open to read it. We also get a lot of clients who make collage boards out of flat cards for various things like anniversaries, airbnbs, or birthdays.
My personal feeling about most things is “less is best” and that holds true with flat cards. Our collection of flat cards is WAY bigger than folded because they are so popular and I find them way more fun to design. Find something that works for your style!