✨ ベストアンサー ✨
全体的にはよくまとまっていると思います。
以下の修正をすれば、更に良くなると感じます。
I think that people will stop sending New Year's cards in the future. 〇
I have two reasons for this. 〇
First, today people are very busy so they have …. → First, today people are very busy, so they have ….
Second, if you stop sending New Year's cards, you do not have to buy it so you can save money.
→ Second, if they stop sending New Year's cards, people do not have to buy them, so they can save money. ※theyに統一
【別解答】
Yes, I do. ← Do you think …? の質問に明確に答える
I have two reasons for this.
First, today people are very busy, so they do not have much time to write New Year's cards.
Second, stop sending them will save people some money. ← New Year's cards の表現を1つ減らす=同じ語句を何度も使わない
For these reasons I think people will stop sending New Year's cards in the future. ← 最後に自分の主張を再度明確にする
参考にしてください。
ありがとうございます!