[SalesForce] Wrapper class, Rest and json

While you could write Apex code that does the search, but the round trip back to the server makes for sluggish UI.

Instead I suggest you do the search in the browser by turning the table you are presenting into a DataTables table (or pick some other equivalent JavaScript library). This also gives you other features such as pagination. The UI is very responsive because the work is all done in the browser.

Google "datatables salesforce" and you'll find blogs on the subject.

You can create ContactWrapper class which contains standard Contact as a field and list of customers.

apex:

@RestResource(urlMapping='/Order/*')
global with sharing class OrderWebServicesV2 {

    private class OrderWrapper {
        Account account;
        list<ContactWrapper> contacts;
    }

    private class ContactWrapper {
        Contact contact;
        list<Customer__c> customers;
    }

    @HttpPost
    global static String doPost() {

        OrderWrapper container = (OrderWrapper)System.JSON.deserialize(
            RestContext.request.requestBody.tostring(), 
            OrderWrapper.class);

        Account acc = container.account;
        insert acc;

        list<Contact> contactsToInsert = new list <Contact>();
        for (ContactWrapper wcon :container.contacts) {
            wcon.contact.accountId = acc.id;
            contactsToInsert.add(wcon.contact);
        }
        insert contactsToInsert;

        //now wrapper list contains contact ids
        list<Customer__c> customersToInsert = new list <Customer__c>();
        for (ContactWrapper wcon :container.contacts) {
            for (Customer__c customer :wcon.customers) {
                customer.Contact__c = wcon.contact.id;
                customersToInsert.add(customer);
            }
        }
        insert customersToInsert;
        return acc.id;
    }
}

JSON:

{
    "account": {
        "name": "Wonderland"
    },
    "contacts": [{
        "contact": {
            "LastName": "Alice",
            "Email": "Alice@test.com"
        },
        "customers": [{
            "lastname__c": "Mad Hatter",
            "email__c": "Hatter@test.com"
        }, {
            "lastname__c": "Cheshire Cat",
            "email__c": "Cheshire@test.com"
        }]
    }, {
        "contact": {
            "LastName": "Queen",
            "Email": "Queen@test.com"
        },
        "customers": [{
            "lastname__c": "King",
            "email__c": "King@test.com"
        }]
    }]
}