A couple of newlyweds prepare to put love letters into a mailbox at a post office in Beijing's Haidian district on Sept 9. The letters will arrive at their home in seven years' time. Pu Feng / for China Daily

Since Sept 9 in Chinese the date is called jiu jiu, which means forever customers have been able to buy special stamps, postmarks, postcards, envelopes and even a Love Passport, which can be stamped every anniversary.

A new zip code 100099 will be written at the bottom of all letters and postcards.

"We came up with the services not only to expand our business but also to offer the public another way to express their love," said Liu Jingmin, manager of the post office, which is near the west gate of Yuanmingyuan Park in Haidian district.

The seven-year mark is commonly believed to be a critical time in any relationship, as it is when eyes tend to wander.

Some of the services, which were jointly launched by Beijing Post and the capital's civil affairs bureau, are already proving a hit with customers, particularly students.

"It's fun and romantic to send a love postcard," said Peng Shuoxuan, a senior at Beijing Forestry University who was writing a postcard to a friend in Zhejiang province. "I love sending postcards in my spare time. The postmarks are especially unique among all the ones I've sent or collected."

Zhang Di, a stamp collector who works at the Chinese Academy of Social Sciences, added that the romantic postmarks "add to the number of varieties worth collecting".

Love Passports are also proving popular, said manager Liu. "A senior couple came in yesterday morning to collect the postmarks on their Love Passport. The little red book contains all the journeys they have made in life," he said.

Although many people said the delayed letters are an interesting idea, others pointed out a potential flaw in the plan.

"It'd be more than depressing if I received a love letter from seven years ago and I was no longer with my then-loved one," said Sun Lubin, a graduate from Tsinghua University.

However, Liu assured: "No matter what, the sweet moment of taking an oath is worth recording, even if people break up years later. Hopefully, when the letter arrives seven years later the couple will receive it together."