1. Appropriate amount: 5.20 yuan, 13. 14 yuan, 9.99 yuan, 99.99 yuan, 7.77 yuan, 77.77 yuan. Common amounts are 13 1.4 yuan, 520 yuan, 1040 yuan, 13 14 yuan, 2099 yuan, etc.

2. It doesn't matter how much it costs to send a red envelope on Valentine's Day, but the mind is the most important. The specific number of red envelopes to express love depends on the individual's tolerance.

3. On Valentine's Day, many couples will use 1 14 to express their love, because 13 14 means a lifetime, and you can send 13. 14 yuan or transfer13 to each other.

4. If you want to shine for each other, you can send a Valentine's Day red envelope of 3 1.07 yuan, which means love together.

5. If you want to express eternity, you can send 9.999.99 as a Valentine's Day red envelope to show that your love for TA will never change.