I would like to convey in this letter my heartfelt thanks to you for...
I am writing to extend my sincere gratitude for... I am writing to express my thanks for...
I feel deeply indebted to you and I really dont know how to thank you enough for your help.
I must thank you again for your generous help. I am most grateful for your selfless donation. My true gratitude is beyond the words description. I feel most obliged to thank you once more. Please accept my gratitude, now and always.
I am writing to show my sincere appreciation for...