Summary: Saying someone thanks with pure heart is good and if writing a letter for this context then off course its best option. A thank you letter should state the reason as to why you are thanking the person. It would be written with kind and lovable words, which allow the reader to know the importance of the gift to the person gifted. The below letter will give you an idea to write the letter.
Mrs. Mary Andrews
85 Lawn Avenue
Block No 79
Dear Mrs. Andrews
I was very pleased to receive the little flower plant presented by you last week. As you know, this is the one I was searching for about 6 months, but could not get it. I was surprised to see the plant in your hands and I am so thankful to you for this kind gesture. You have been so thoughtful to get this plant for me.
I am very touched by your lovely present. I will take care of the plant like my child and you will be happy to see it grow in my garden as good as it can be.
Thanking you all over again for the thoughtful gift.