The Wingswept Sky. Dragons of Amontyr, #1

Kayleigh Espinosa wanted nothing so much as to be a courier, running messages through the streets of her home, tracing her maps with her own two feet. But on her first day, her hopes are dashed when the Courier Guild's newest transport dragon chooses her to be his rider. Only, Zapyros isn't the usual transport dragon. He's a wardancer, bred for combat and transferred into the Guild after losing his last rider.
Now, Kayleigh must figure out what it means for to be a courier and a dragon rider. When her training master takes a particular dislike to both Kayleigh and Zapyros, she takes matters into her own hands to be the best courier the Guild has ever seen. Facing dangerous flocks of dragons and lightning-filled storms, being a courier is more perilous than she thought. With new friends and a dragon at her side, Kayleigh takes to the skies in the Great Race, the Guild's most anticipated competition.
And she intends to win. The Wingswept Sky is the first book in the Dragons of Amontyr cozy fantasy series. It features a neurodivergent main character, found family, and of course, dragons delivering the post.