That is not possible with Airtable’s native forms, but that can be accomplished with Fillout’s advanced forms for Airtable. I give a demonstration of this on this Airtable podcast episode.

— ScottWorld, Expert Airtable Consultant Since 2018

Thanks @ScottWorld . I will check the resources you shared

Thanks @ScottWorld for the resources you shared. Your demo on fillout on picking multiple products with their quantities  and their amounts sumed up is exactly what I would love to achieve in my ongoing project. 

However, you didn't explain in that video, how you added those features (multiple products, quantity and their total sum)

I'm stuck stuck as seen in the attached picture.

I am a beginner please could you make the explanation easier to understand?

I don't have step-by-step instructions, but here are some screenshots on how I set it up. Fillout also has excellent technical support & excellent instructional videos on their website — so I would definitely reach out to them if you need more help.

Thanks @ScottWorld , the screenshots are helpful. Can I get a similar screenshot for adding quantity to each product when multiple products are selected? 

I will surely check out videos on the fillout website. Thanks for your help.

This is the child form which handles the line items table: