Hongkong sends postcards to the mainland.

you can write the address directly in simplified Chinese characters, and the postage for sending it to China is about 1.5 Hong Kong dollars (this is the standard in 25, which may be increased a little). Postcards can be bought in bookstores and ordinary tourist shops. Some postcards are printed with postage, so you don't need to affix stamps. Those without postage or written with "postage limited to Hong Kong" need to affix enough stamps. The post office is closed on weekends, so you can write it and give it to the front desk of your hotel for mailing, or directly drop it in the nearest pillar box.

I hope I can solve your problem.