To buy a single item on 1688 first I'd suggest you just change the order quantity to 1, order and see what happens. If it works you're good.
The alternative is to change the quantity to 2 and edit the price to half. Then write a remark to your agent that you only want one.
If there's a problem your agent will surely tell you. And then you can still ask them to negotiate for you.
added: 6/26/2023