This sounds like a fandom thing. Maybe your fandoms are popular on Tumblr and that’s where your authors are being sourced from, so they expect that from you.
Regardless, if your letter is long then they just need to suck it up. If you want to meet them half-way, maybe lurk the tumblr tag and link posts with gifs/meta for your favorite moments or add some tumblr-tag style AHHHHHHHHH THEY’RE JUST SO SMART AND CUTE AND??? KISS ALREADY. I LOVE HOW X STARES AT Y’S LIPS LIKE OMG WE SEE YOU FROM MARS id rants in your letter that I guess they feel like they’re missing by not being able to lurk your tumblr tags?
Re: Is an AO3 account not enough?
Regardless, if your letter is long then they just need to suck it up. If you want to meet them half-way, maybe lurk the tumblr tag and link posts with gifs/meta for your favorite moments or add some tumblr-tag style AHHHHHHHHH THEY’RE JUST SO SMART AND CUTE AND??? KISS ALREADY. I LOVE HOW X STARES AT Y’S LIPS LIKE OMG WE SEE YOU FROM MARS id rants in your letter that I guess they feel like they’re missing by not being able to lurk your tumblr tags?