I want to buy something in Amazon, but I want to ask how to contact the seller (the third party). Where is the mailbox entrance?

Hello, as shown in the figure, you click on a commodity first, and then you will see that it will be sold from XXXXX (store name).

Click in, and you will see the whole shop page, in the lower right corner, as shown:

Click to contact the seller and contact the seller by email.

That's it. You can try it. I hope it can help you. I am an American seller of Amazon.