Add in MYSQL query in varchar field by removing Strings

You need to add the converted value:

• remove the R $,
• remove point from thousands
• Replace the comma by period

After that convert to number and add:

SELECT SUM(
cast(
  REPLACE(
    REPLACE(
       REPLACE(valor_total, 'R$ ', '') /* REMOVE R$*/
      , '.',''),   /* REMOVE O PONTO */
     ',','.')      /* TROCA A VÍRGULA POR PONTO */
        as decimal(8,2))
)  AS TotalDaSoma
FROM carrinho 
WHERE representante_id='92'

I broke into lines to make it easier to understand.

Here's a fiddle showing the conversion working: link

It is necessary to chain the functions, leaving the one that must be executed first within the one that should be executed last, in your case it would look something like this:

SELECT SUM(REPLACE(REPLACE(REPLACE(REPLACE(valor_total, '.', ''), ',', '.'), ' ', ''), 'R$', '')) AS TotalDaSoma
FROM carrinho
WHERE representante_id='92'

In this way the following functions will be performed:

• Replace . with nada ;
• Replace , with . ;
• Replace espaço with nada ;
• Replace R$ with nada ;

Click here to see an example working.