Hope your holidays went well.
"CustomFields are not returning when i tried with MethodAPIFieldList which will return list of fields for the Customer table..(According to method documentation, we can post the values to the fileds, which is returning by the API - MethodAPIFieldList"
-If there aren't any custom fields coming in, chances are the custom fields don't even exist in QuickBooks at all. If you have custom fields setup they will should show in the API just fine. They have to be setup in QuickBooks first if you want to use QuickBooks custom fields. (Customer > Additional Info > Define Fields).
-Without knowing what you are trying to pull in with Estimation values, it's hard for me to see if you are heading down the right path. Where exactly in QuickBooks are you currently seeing these values that you want to get access to with the API (which screen, how do you get there, etc.)?
"And another thing is like, when i tried to pass some grandtotal value to TotalBalance field in Customer table, its not allowing to insert because TotalBalance fields are not supporting for edit or add a value in to that, because Default setting for the TotalBalance fields is set as False"
-Yes, this is true. TotalBalance is a read only field. It is calculated by QuickBooks. As per my previous response, I suggest the programmer checks into the QuickBooks SDK (a link to which is given in the Method API docs) to learn more about why certain fields are read only.
"so, only string values can be able to insert using MethodAPI.. only the customer information can be able to insert using MethodAPI.."
-No, this is not true. Just because TotalBalance is read only it does not mean that all non-string values are read only. It just means that TotalBalance is a calculated, read only field (it is calculated based on the totals of all invoices, statement charges, credit memos, payments, etc, that hit the account). The vast majority of date, string, and number fields all support edit and add.
-If he wants to affect the total balance, he needs to create a transaction against the customer that will affect it.