First, When I create a new Invoice with InvoiceLine the Rate is set as 0.0 although I set a value on it ! 
this is the body request:
{
    "TxnDate": "2020-11-12T16:00:00",
    "BillAddressAddr1": "Address1",
    "BillAddressAddr2": "Address2",
    "BillAddressCity": "City",
    "BillAddressState": "State",
    "BillAddressPostalCode": "00000",
    "BillAddressCountry": "USA",
    "IsPending": false,
    "IsFinanceCharge": false,
    "DueDate": "2020-11-12T16:00:00",
    "IsPaid": true,
    "IsToBePrinted": true,
    "IsToBeEmailed": true,
    "IsTaxIncluded": false,
    "IsWaitingForSyncApproval": false,
    "TenantID": 1,
    "ARAccount": "Accounts Receivable",
    "Customer": "Abdullah Shublaq",
    "__InvoiceLine":[
        {
            "Desc": "Invoice Description",
            "Quantity": 2,
            "Rate": 50,
            "IsRatePercent": false,
            "ServiceDate": "2020-11-12T16:15:25",
            "UnitOfMeasure": "",
            "LineOrder": 1,
            "LastModifiedDate": "2020-11-12T16:16:08.597",
            "Item": "Sales",
            "SalesTaxCode": "Non",
            "TenantID": 1
        }
    ]
}
Second, what is the request that I can create a new Item throw it to add a new InvoiceLine for this Item?
Finally, what is the request that I can create receive payment for the invoice to make it Paid?
Thanks.