If I buy an HP laptop in Guangzhou, can I enjoy after-sales service in Zhoushan or Hangzhou?

First of all, it depends on whether you buy licensed or parallel goods.

If you buy licensed goods and have a formal invoice, you should enjoy the national warranty.

If you buy parallel goods (non-mainland licensed goods), or you don't have an invoice, it won't work. This final interpretation right belongs to the merchant.

It's strange. Why didn't you ask when you bought it?

Don't give unscrupulous merchants the opportunity to exploit loopholes. While enjoying their rights and interests, we should also let them fulfill their obligations!