Condition on the result of a calculation

Hello

I am creating a form for direct meat sales.
The sale is made in the form of packages in the number of 5
For 3 of them, you just have to enter the number of packages you want
For 2 others, you must indicate the number of packages but also choose from 3 items: the customer must distribute 6kg over 3 different items
The form must therefore block the sending if the total of the three choices does not add up to 6kg

Is it possible to display a message saying: the total is not 6 (for example using a calculation field)?
And therefore prevent the customer from going further

I tried the method of hiding the send button if the total is not 6 and it works. Problem if the customer simply wants another package, the send button does not appear …

What method do you suggest to me?

Thank you for your help

Freddy