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 don't 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 word's 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...