The best alternatives to Hatched are Capsule, Letter From You, and Vuash. If these 3 options don't work for you, we've listed a few more alternatives below.
No machines. Handwritten by real people. Wax sealed. Take some time to write a sincere letter. Write your clients how much you appreciate their business. Ask how you can better serve them. Earn their trust in you.
Write a letter to your post-quarantine self to receive at a time when you believe all this will be long behind. Remind that future self the lessons you learned during this time, and hold them accountable to do the things you had missed so much.