Dear [Friend's Name],
I hope this letter finds you well. I wanted to take a moment to express my heartfelt gratitude for the thoughtful gift you sent me for my birthday. Even though you couldn't make it to the party, your gift truly made my day special.

Appreciation for the Gift:
I was pleasantly surprised when I opened the gift you sent. The [describe the gift] is something I had been eyeing for a while, and your thoughtful gesture truly touched my heart. It was such a thoughtful and meaningful present, and I am incredibly grateful to have you as a friend.

Understanding Your Absence:
While I missed having you at the party, I completely understand that sometimes things come up and plans have to change. Your presence was missed, but your gift made me feel your presence in spirit.

Importance of Your Friendship:
Your friendship means the world to me, and I am grateful for the bond we share. Your kindness and thoughtfulness never fail to amaze me, and I feel truly blessed to have you in my life.
Once again, thank you from the bottom of my heart for the wonderful gift. I look forward to catching up with you soon and celebrating more special moments together.
Warm regards,
[Your Name]