Do you want any additional things?

[Winwin2011]

Shop assistant: Do you want any additional thing (singular) ?
Customer: No, thanks, that's it for now.

I wonder if the following conversation is correct:

Shop assistant: Do you want any additional things (plural)?
Customer: No, thanks, that's all for now.

Is it correct for us to say " that's all" referring to additional thing or additional charges?

Thanks.

 

[Raymott]

Shop assistant: Do you want any additional thing (singular) ?
Customer: No, thanks, that's it for now.

I wonder if the following conversation is correct:

Shop assistant: Do you want any additional things (plural)?
Customer: No, thanks, that's all for now.

Is it correct for us to say " that's all" referring to additional thing or additional charges?

Thanks.

A shopkeeper is never going to say that. He might say, "Do you want anything else?"
Yes, "That's all" is OK as an answer.